I wear a size 11B.  Some boots I have found I can surprisingly wear a 10 or at least try it on for general look and feel then order an 11B.  This boot fit like a 9.5.  It is comprised of very thin material, obviously not real for the price, but very cute.  They can be folded down for the knee height look. I would say it runs 1.5 sizes small, and would work for a costume if you could get a size big enough (1.5 sizes larger than your normal size), but I can't see it standing up to regular wear.  I also bought a pair of Chinese Laundry thigh highs for about the same price.  Those 11's fit great.  They are comfortable and sexy.  I could even wear thin-average thickness socks with the Chinese Laundry thigh high boots, and they feel more comfortable to stand in.  I will get a lot of wear out of the CL's.