Ok i know they come in MM sizes, but for the sake of arugement, lets just you
Thin, Medium, and Thick.

I usually use medium, but I picked up a really really thing pick I found lying around my room and started playing with it and it was really different. I dont wanna say better, although in some ways I thought it was....but anwyayz

I was just curious as to what the different sized picks are used for and their purpose. is one used more by a beginner? Or do they have certain qualities in them, something more then just preference.

But i guess things like electric guitars cna use lighter picks then acoustic and stuff too. Just curious on others thoughts.
it's all based on personal preference. some people I know like larger, very soft/flexy picks, when I myself like smaller, harder picks. It all depends on the player and what he/she feels more comfortable with.
They say that heavier guaged picks are supposed to be good for pinch harmonics, but the way i see it (so far) is if you get good at the technique then it shouldn't matter what guage you use.
i find the thin picks to be god for strumming like acoustic strumming with alot of chords although i use thick picks cause i play rhythm and i get mroe of a metal sound but medium picks i find good for both picking single ntoes and strumming chords
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For Acoustic I almost always use a thin pick, for electric I prefer heavy. I'll use medium for either one.
i like thin picks for precise movements like soloing and stuff, but thicker harder picks for heavy strumming like punk songs
Just a matter of preferance. If you want to produce Pinch Harmonics a little easier you use a Heavy Gauge pick. Its more of a challenge with a smaller pick. But, once you get the technique down its easy across the board. I prefer the George Dennis Green Sharp .88mm
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the different sizes of picks, the first noticable thing you will see is how they feel across the strings. i use thick (1mm) picks for punk and stuff that requires precision. just because i need to hit hard and the pick has to follow my every move.

thinner picks are generally better for strumming because they just move through the strings with much more ease.

then there's the physical size of the picks (not thickness), which make it easier for strumming. the small teardrop ones i've seen jazz musicians use because its more precise.

then you can have the super thick picks (3mm), and naturally these kinda soften the strike on a string and provide a slightly muted sound.

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I use thin for pretty much everything (red dunlop .50mm tortex if anyone cares), and I seem to think it gives the notes a prettier, less harsh sound, which is what i like.
I use credit card thickness and above.

Now I use jim dunlop 2.0mm dunlop picks, those big triangle ones, I also occassionally use jazz 3.0mm picks, and Peavey 1.0mm picks. Yea, I love thick big picks.
I prefer medium thickness. Usually about .88 mm. Sometimes a little less.

I think that medium picks give the best of both worlds. Thick enough to have accurate picking, but thin enough to get good strumming.
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yea i have a friend who plays and is really. But he swears by it that the only way to be fast is to use to stubby jazz picks, lol
i love my 2mm big stubbies. they do make pinch harmonics easier, as well as speed runs, since they're so small.
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thin picks are good for soft strumming, thick ones are good for hard strumming.
pointy picks are good for lead and rounded picks are good for rhythm.

use whatever feels comfortable for whatever you're doing. it isn't rocket science.
I use my Red Jazz 3s for Everything Bar struming out some simple chords on the Acoustic for the people who don't like the metal I usually play
i see a lot of people with the jazz picks, im gonna check next time I go to the guitar center, which will be this weekend! woohoo, im gonna get me a new guitar!

But i dont remember seeing those picks last time I went
.73 Dunlops. Only picks Ive used for about 8 years now. Anything thicker gives too harsh of a sound, but anything thinner makes it sound too weak.
I use medium-thick on both electric and acoustic. I find that if you play softer a hard pick can sound softer but not the other way around. I bought like $20 worth of picks off ebay recently its sweet, they came in a big bag of all shapes, guages and colours and you can try each type out to see what you like best.