l am currently building a fixed gear bicycle and i'm not in that great of shape.What would be a good sized gear for someone like me?

There is a difference between the crank arm size, & the gear size. Which r u referring? Crank arm size is more related to ur bike fit; like how long ur legs r instead of how strong. You really want a crank size that fits u the best, then worry about choosing the right gears for u strength. You can always swap the gears easily. The crank arm length for me at least, is 172.5 mm. I''m 5'10'' with average leg length. The other common sizes r 175mm, 170mm, 165mm.

Try looking into this site, build a bike on the website to get a good idea what u expect. It is pretty fun anyways as well.

http://irocycle.com/

Ron is right there is a difference between crank size & the gear ratio. Road bikes usually come with a 170 mm length crank arm & mountain bikes may have a 175 mm arm. The gear ratio depends on the terrain u ride & ur physical conditioning. If u r starting out u want to get a multi speed bike before u decide on a particular gear ratio. One gear ratio may be good this week but be too low next week. Most fixed gear riders will know what gear ratio they need for their daily routine. This link may help.

http://sheldonbrown.com/fixed/index.html

l am assuming you meant gear sizes, or at least do now after the other posts. If you have a road bike try riding it without changing gears for awhile. That will give you an idea of what gear you want to ride in.

If u do not have ur rear wheel yet, think about getting a flip-flop hub so u can have two different ratios (which is what l am guessing u mean). You can get one with either a track threading on one side & freewheel threading on the other, or track threading on both sides.

In addition to Sheldon is site, check out the Rabbit gear calculator. Link is below. l downloaded it & have it on my desktop for handy reference. You input chainring & cog sizes, length of ur crank, tire size, & whether u skid (if u do at all) ambidextrously, & it tells u lots of useful stuff like gear ratio, gain ratio, gear inches, development & has a nice table that shows what speed u will be going at various rpm's. It even tells u how many skid patches u will have on ur rear tire if u skid.