Okay, i have a 2000 Chevy Silverado and am planning on putting 20's on it. But the thing is, I want to know what's the biggest size of tires you can put on 20's?? I like the look of the big rims, but not the look of the small tires. So im trying to incorporate both. Any help would be appreciated!! ;)

285/50/20 is the recommended size for that truck, which doesn't look small at all.

The following sizes will work depending on the wheels you purchase and how they are set up.
265/50-20
285/50-20
295/45-20
305/45-20

It also depends on which version of the trim package you have on the truck. When you are looking at the wheels, ask what tire size they have actually test fit with that rim. If they can't tell you that, they have not test fit the wheels on the truck.