I think the best way to figure out what gear you need is by riding around on your geared bike and find the highest gear you can still crank over the hills.

Then punch in those gear numbers (53x23 for me on 700cx23 tires, I live in a really hilly area) in sheldon brown's gear calculator for gear inches, then find a corresponding gear for your fixie. I'd go a little higher than your actual gear inch because on your geared bike, you know you can shift to an easier gear and it holds you back from cranking over the gear.