I ride a 46x16 and a 48x17, that seems pretty good to me, but Albuquerque is not super hilly within the city (not flat either). I found that anything higher than 48x17 got pretty bad in a strong headwind. Use a gear inches calculator to get an idea of how much of a difference these would be from your current set up. As someone else mentioned, if you have a road bike, find cog/chainring combos that match the gear inches/gain ratio of the fixed ratio you are contemplating, and try it out.

I've used IRO and Surley cogs, both with good results. Be sure to get the correct width cog, in my experience Dura Ace 1/8" cogs are a little thinner than others, and can be mistaken for 3/32" by some know nothing LBS workers.

On my bike that has a freewheel, I went with the same size as the cog, because I enjoy that ratio. I think it would be more practical to make it a little larger, since you'll be using it when you're tired.