how big for stock 06' rancher

I would go no bigger than 26s, 10s & 12 or 8s & 12s.

26x9.5x12 vamps on front
26x12x11 vamps on back.

unless you want to get different wheels.
if not these would be my choice

oem is 24-8-12 and 24-9-11. IMO every inch larger in height is a knock down of 5% on power and an increase in speed of the same. 2 inches wider than stock is usually maximum and any more than 2" in height needs a lift kit to not rub fenders. I run 25-10-12 on front and 25-10-11 on rear with no issues as to clearance , but to run mud in 2nd gear, I had to install a dynojet kit and hi flow air filter.

a friend of mine has 27 itp 589 on his but he don't have hardly enough torque to pull em on his 400at

The wife has 25x8x12 and 25x10x12 tires on her 04 Rancher and likes them. I wouldn't go any higher than 26, otherwise your looking at some mods to your bike.

I also think you should go with 26"

26" would be the biggest i'd put on a rancher

I think something like a 26x9.50 would look really good all the way around. That way you have less weight to bog you down too. This is what I considered doing when I get Vamps. Any input on this?

when i had a rancher i had 26x9 superswampers on all four coners and that was perfect. i could follow my cousin who had a rubi with 26 outlaws with no problem.

on my dads we got 26*13 in the rear vampire asx didnt turn them so well until we put the hmf on now it spins them like crazy