265/50-20 fits ~7.5" - 9.5"
285/55-20 fits ~8.0" - 10.0"
However, 285/30-20 fits ~9.5" - 10.5"
And, 265/35-20 fits ~9.0" - 10.5"
These numbers are for an example tire, the example being BFG KDW-2's. As you see, the smaller the profile, the wider the rim must be to accommodate the smaller sidewall. So using this as a rough estimate, 285/50-20 (which isn't offered @ TireRack atleast) would be recommended on 8.5"-10.0 or something similar. This isn't all-inclusive though and you can stretch almost any tire of appropriate inner diameter onto a wide range of rim widths.
I put some rubberband-like 255/35-20 tires on my stock 20's just to see how it looks. Didn't like it so I'm gonna sell them, but that just goes to show you since it is recommended 8.5" - 10.0", you can still fit it just fine.
So in summary, I'd go 285/50-20 if you can find that size. The sidewall would be .4" taller and overall tire diameter would be .8" taller than stock 265/50-20, or 2.6% larger resulting in 2.6% slower speedo and 2.6% less mileage on the odometer.