In the wonderful guitar universe, it’s important not to criticize what size strings a man or woman uses. However, any guitarist that has spent just a small amount of time on internet forum and message boards has navigated the minefield of these types of threads. This article will explain the reasons why someone might choose a particular string size, and unraveling the myths and truths about what it says about the player him-or-herself. 
In My Day…

Most vintage strings didn’t have any gauges listed.

Actually, it was before my day, but many years ago, before Leo Fender turned guitar making into an efficient assembly line process, electric guitars were basically acoustic guitars with pickups. Strings sold for electric guitars were essentially acoustic guitar strings, and acoustic guitar strings were big. There was a practical reason for this: acoustic guitars needed big strings to be heard above a piano and drumset in the dance bands of yesteryear. Solos by guitarists were rare, until the dawn of the electric guitar. The electric accessories market was non-existent at this point. Most strings were sold with just the string number on them (strings 1 through 6), and certainly not precise measurements of to the thousandth-of-an-inch like we are used to now. String manufacturing hadn’t gotten that precise yet, and if anyone is “lucky” enough to try a guitar from 50 or 60 years ago with the original strings will be amazed that anyone could play these things at all.

A young Ritchie Blackmore before he made it in Japan.

It wasn’t until the mid 60s when guitarists like Ritchie Blackmore started replacing the standard guitar sets high E string with a thinner banjo string and moving the rest of the strings over, kicking off the rather large low E that the idea for the modern, smaller sets of strings was born. Remember, the G strings of electric sets were wound (from the strings’ heritage on the acoustic guitar), and the day’s modern bend-centric style of playing was much easier on a guitar with a plain 3rd string.*
*Vintage single coil pickups have their polepieces staggered to balance out the volume of the strings, and were designed thinking you would have a wound G string. Replacements for these vintage pickups are available from Seymour Duncan as the APS-1.
Acoustic guitarists traditionally used bigger strings for pure volume. Bigger strings drives the sound-producing top harder, resulting in a sound that still be heard over multiple instruments.
So, what happened with all these big strings?

It is so much easier to amplify the acoustic guitar these days with the Acoustic Tube pickup.

The need for bigger strings for volume stopped being a concern when amplification got bigger. On electric guitars, amps could take the smaller output from thinner strings and amplify them just as well as they could the bigger strings.
Styles of playing changed, with guitarists bending strings two whole steps (or more) and that simply wasn’t possible on thicker strings, at least not without pain. In the 70s, higher output pickups started coming on the market which could take thinner strings and goose the signal going to the amplifier.
For acoustics, acoustic pickups started becoming available, and performers who were uncomfortable with big acoustic guitars with large strings suddenly figured out that, when plugged in, the sound isn’t always coming from the top of the guitar vibrating when driven by large strings. Styles like fingerpicking could now be heard, and smaller strings didn’t chew up nails or fingertips as much.
Also, string manufacturing took great leaps forward, with companies discovering new alloys for strings which produced more volume, with greater more consistent size tolerances.
The Case For Big Strings

SRV popularized the use of Very Large Strings in electric blues.

The big string craze may have started back in the 80s with Stevie Ray Vaughn’s interviews in guitar magazines. Stevie used big strings (reportedly up to .013 on his Strats, but not all the time), and he tuned down 1/2 step. Tuning lower will make thicker strings easier to bend with the decreased tension. Stevie was also a pretty heavy player, and he beat his Strat into submission every night. He used vintage gear and vintage output pickups. No doubt that contributed to his bold sound.

The Jericho Avenger is a baritone guitar with larger strings and a long scale. It also comes stock with Seymour Duncan pickups.

Modern styles require low tunings, and anyone who has ever taken a .042 E string down to C# knows that it’s like playing a guitar strung with linguine. The strings are not tight enough to provide that tightness in the bass, and sustain is lost. Bigger strings help lower tunings sound brutal and mighty, and on 7- and 8-strings, the gauges get bigger and bigger. To be fair, most of the styles that use those guitars aren’t concerned with bending the big strings that frequently.
Also, scale length is a factor. To me, .009s on a Strat feel like .010s on an LP. A Les Paul’s smaller sounding length of the string itself (the scale length) will have more of an effect on feel due to the tension of the strings. In general, if you like the feel of .010s on a Strat, it will feel similar to .011s on a Les Paul.
As far as tone goes, large strings on a long scale (26″+) guitar sound positively massive.
The Case For Smaller Strings
Well, this one is easier. Smaller strings are easier to play. You can bend them, add vibrato, divebomb ’em, and it’s all easier. Besides, Billy Gibbons uses .008s. So does Yngwie Malmsteen. Need something heavier? How about the Iron Man himself, Tony Iommi. He is proof that with the right rig you can get downtuned skinny strings to sound positively massive. Tony uses a lot of gain, high-output pickups and a very light technique. And again, it rocks.
http://www.youtube.com/watch?v=vcWn_IrqBPc
Where I Stand

This *can’t* be how my strings are made…

My guitars are all 25.5″ scale, and I use .009s. I’m a really light player, and I rarely use more than a light overdrive. I mostly avoid power chord-based rhythm, and .009 works for me. In other words, I don’t want to work any harder than I need to to get the job done, and I am very conscious of having a relaxed playing style so I don’t end up in pain So, .009 is the right choice for me.
On acoustic guitars, I use .012. I mostly need the guitars to sound good when plugged in and recorded with microphones in the studio, and .012 seems to be a good balance between ease of play and tone.
What Should You Choose?
If you haven’t been playing for long, it might be a discovery process. You can make anything work, but you may have to try a few different gauges over the first few months of playing to settle on something that gives the right balance of tone, volume and playability. You can always investigate the tone of your favorite players and see what they use, and that might be a good place to start too. Just remember that going up or down in string gauges might cause your guitar to need a few adjustments. Don’t worry, they aren’t difficult.
String thickness is a personal choice. Certainly some people can easily bend heavy strings, and other people can get a positively huge sound out of thinner strings. Use what you like, no matter what the other people say on the forums.

What string gauges do your favorite guitarists use? Do you have a favorite set you use for all of your guitars, or are they all the same?