if you want GM wheels you could go with a set of the 17" wheels that are standard on the colorado Sport model. Or the Intro Prowler 6 wheels look pretty good I think. They come from 15" to 22". I'm considering a set of 18x8 or maybe 19x8 for my colorado.
I think with the 4 cylinder engine, it might be best to use aluminum wheels (not steel), size 18" or smaller to keep the weight down. As far as the speedo or anything being off, I don't think it would be anything major to worry about. Main thing, as long as you can still turn sharp enough, go for what looks good to you I guess.
link to Intro Prowler 6.
http://shop.sportruck.com/intro/6.htm