Measuring a fixie/single speed bike
Fixie/single speeds have the same measurements as any other bike. 

Most fixies/single speeds have geometries that are like old road bikes or hybrids depending on how they're built (obviously a fixie built from an old road bike will have an old road bike geometry!). So, you need to take that into account and preferably try the bike before you buy it. You may find you want a longer top tube or shorter top tube or whatever.