Somebody help me out.

I am not sure about how carburator mods work on 4-stroke motors.
Basically;
1) What does the size have to do with performance? i.e. 27mm, 34mm, etc...
2) Is it wise to increase size with a stock motor? Any gains, benifits, pit-falls?
3) Case Study: What if I replaced a 27mm carb with a 34mm carb?

Reason for asking, I need a new carburator for my ATC 250SX. Rather than put a stock carb back on it, I am curious if there is something to gain by swapping for a better set-up. I do plan on modifying the engine at a later date and a K&N filter will be put on real soon.

Thanks.