I just did a 6" pro comp lift now I'm looking at 35" tires
What size wheel should I go with? 18" x ?

 

What I'm asking is do I need a 10" wheel and what backspaceing

 

A lot of 6" lifts require at least a 20" rim. I'd go with a 20x9 with 0 to -12 offset.

 

Does the diameter of the rim really matter? I've seen dude running 17s. I know width, backspace, and offset are issues

 

Yes, on some big lifts, the steering knuckle is a lot bigger and anything smaller than 20" will not clear it. Check specs on the kit you got.

 

I won't be buying one for a while, but I appreciate the heads up.

 

I'm running 18's now and it's ok

 

Then 18 it is! I prefer the look of a 35" tire on an 18" rim, makes it look more beefy. 9" wide is pretty common. 10 almost makes the rubber balloon out too much.

Backspacing / offset is all personal preference - you'll have to decide how much you want it to stick out past the fenders.