Originally Posted by
comma
The purpose is that this bike fits me well and I want to know the frame size for when I get something new.
Finding an old Trek catalog can't be the only way to measure my frame size...can it?
Like the other poster said, it's typically measured from the center of the bottom bracket to the top of the seat tube. Looking at Trek's catalogs back to 2007 or so, they list 16" and 18" frames among the sizes for the 3700. You can be fairly certain that's a 16.
More importantly, from Trek's specs you could find other (more) important dimensions, such as effective top tube length and reach.