How big can I print with the D90 and still make it look good?

I frequently print 20x30 with my D300 and I've even printed at 40x60 and had no issues as long as I was careful with in camera and post techniques.

My "other" simple answer - Upload a photo to Costco and for $9.00 get it printed at 20x30. What do you have to loose? It may take an hour or so but if it works out you'll have a great piece of work. If not, ask yourself "why"? What could you have done better?

If 20x30 looks good - try 40x60

Good luck!