An interesting question.

The frame size that is most frequently qouted is seat tube length. It's most commonly measured from the center of the bottom bracket shell to the top of the top tube but there are at least a couple more different ways that it's sometimes measured.

The really crazy thing is that knowing the seat tube length isn't really all that helpful. Saddle height is the easiest adjustment to make on a bike. Many riders feel that top tube length is a more useful dimension for determining what size bike to buy. Since tall people tend to be tall all over, bike frames tend to get longer as the seat tubes get taller.

That's a long way of saying that, although you may physically be able to straddle a 61cm frame, the handlebars will likely be too far away from the seat and you'll never be able to adjust the bike so that it's comfortable for you to ride.

Your fit on a road bike will affect your comfort, efficiency and performance every minute that you're on the bike. Nothing else comes close in importance.