What are the dimensions of a twin size bed box spring?

Twin sized. The problem is that this isn't defined in any consistent way. The best solution is to just bring a measuring tape, or tell the person telling you it's a twin "I don't care if there are two of them, give me measurements in inches. Or centimeters. Or something."

You could also go to the link in the related links for a discussion of what the various ridiculous bed sizes mean in various places.