I'm usually good with bikes with horizontal top tubes. Once they start sloping, then it's just a crap shoot for me. I tend to judge my size (50) by how far away the top tube is to the rear wheel. Yeah, it doesn't work for all bikes, namely ones with smaller than 700 wheels. Then I start estimating them way bigger than they are. But it sure does make craigslist-tabbing much faster when I'm looking for a bike my size.