First, get familiar with Sheldon Brown's online gear calculator:

http://sheldonbrown.com/gears/

Enter the correct wheel and tire size.

Enter the correct crank length.

For gear units, select gear inches (not gain ratios).

Just as a starting place, put 47, 48 and 49 in the chain ring windows, and 16, 17, 18 and 19 in the custom cassette (cog) windows.

Hit calculate, and notice that 48X18 gives you 72 gear inches and 49X19 gives you 69.6 gear inches.

I find 72 gear inches the most all around versatile ratio, although I have happily ridden as high as 82 inches and presently ride 77 inches.

I think 69.6 inches would work in Colorado.
Climbing with a fixed gear involves cadence and form more than brute strength and stamina.
Use clipless pedals, get out of the saddle, and PULL yourself uphill at a nice slow cadence.
You will get better at it as your spin improves.

Do not get a single speed freewheel.
Ride fixed, especially downhill.
At 69.6 inches you won't go very fast downhill, but you will feel in control, which has its own pleasures.

Ride 69.6 inches for a month or more before going higher.

Go back to the online gear calculator and try putting 43 in the chain ring window and 16-19 in the cog window.
Some very accomplished bicyclists of my acquaintance ride these lower gear inches, which make it much easier to go up and down hill - and talk about control.

Just forget about raw speed for awhile and have fun with precision and control.
I promise you will make some very pleasant discoveries.
I hope to never go back to a geared bike, but for an ice bike in winter, I presently have no other choice.
Maybe by next year's income tax refund...