I don't think you should size down for a fixed gear. A lot of people will do it with the excuse that it will make the bike handle a little quicker. I don't really agree. A smaller frame can make it a bit easier to learn to skid, since you don't have to work as hard to get your weight forward. If you're going to be using the bike for commuting, you want to be comfortable, so get a bike that fits.

As for frames, just look for what you would look for in any vintage frame. Right size, decent tubing, fender eyelets if you live where it rains. And of course, horizontal dropouts are important.