Your question is not at all ridiculous. Triangles come in all shapes and sizes, and I would not automatically assume that you meant a HST, but since you do, I would agree with the others who would generally refer to it by the size of the finished square. For 60 degree (equilateral) triangles, there are actually differences in how rulers are made to measure them. Some measure from the base to the tip, and others measure the sides.
http://www.frommarti.com/mm5/merchan...ategory_Code=R