how big can i go

i have a 1999 silverado 1500 4x4...the truck is still stock hight..i have 20x8 rims for it but being 4x4 i dont want low pros on them wat is the biggest tire size i can go

i think a 50 series looks good.. my friend has boss 20x8.5's on his 02 z71 with, i think they are 255/50/20's.. and they look really good on there.

can i go any wider then 255 on an 8 inch rim?