yeah you shouldn't ask them to size you for a bike if you don't intend on buying. no wonder they're being unfriendly! they'll be your best friend if you buy a bike from them!
sounds like you're trying to do what i did and shopping for a used bike online... you need to be extra careful about sizing... i will discuss this below.
go to
http://www.wrenchscience.com for a good geometry calculator. and please remember that not all 50cm frames will fit you the same. I had to go through 2 wrong-sized bikes before settling on my current one.... a specialized 54, 52, and now i'm on my current love, a 50 cm specialized allez comp.
look closesly at geometry specs... most specifically, top tube length, standover clearance, and seat tube length, in that order.