I am going to go against the consensus here and say 20s. I had 22s on my previous truck and they were a bit too big. I think 18s are a bit too small if you are upgrading from the stock 15s or 16s. 20s with a tire like you mentioned would look good I think. Some sidewall to fill the wheel well, but you still get the updated look with the larger wheel. Check out Badger69's truck towards the end of the page
here. He has some decent sidewall on his 20s and it looks badass.