How big can i go on the rear tire, if i want to start customizing a zx14? What am i gaining and what am i losing? Which is better single sided swing arm or the regular swing arm?

You can go as big as your dreams and your $$$ will let you!!!

Tire size (width, profile) affects handling. The handling diffence between a profile 50 and 55, for instance, is noticeable.

An oversize tire in the rear will hurt handling.

As for swingarms, you might save some weight with a single sided swingarm, and it may be easier to remove the wheel.

A fat tire gains nothing except the whacked out showbike look. Adds a pile of unsprung weight which apparently renders any increase in traction irrelevant. WIll not corner effectively like a round, normal width tire. You will be riding up off the flat bottom onto the edge when you lean the bike which seems extremely unpredictable to me(and you prolly wouldn't be doing much leaning, at that).

I always loved the look of a single sided swingarm. I asked about that years ago on the old forum and the answer seemed to be SS swingarms on the 14 were ONLY for FAT tires. No other reason. No big weight loss. No performance improvement. No ZX-14 race bikes have SS swigarms. An extended swingarm won't make the bike handle better either but it may be of benefit for drag racing.

If you are out to build a showbike, who cares how the bike handles? If yu want a bike with optimum performance, you need to decide what kind of environment the bike will be operated in. Drag racing, track LSR, street. This is one where you can'r have the best of all possible worlds.