Originally Posted by Carson Dyle

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.