jackb

My adult son is buying a new Trek Checkpoint. According to the Trek sizing guide his height suggests a 56cm frame but his inseam suggests a 54. Which is the most important measurement? Would either frame size work?

Iride01 

Either size might work. Even sizes outside the suggested range might work.

I'd probably go toward the smaller frame size. But a lot will depend on whether your son will like the slightly more saddle to handle bar drop that the smaller size will have.

Though the Checkpoint doesn't look very aggressive in fit. So it may be better all around to go smaller. But you never know for certain until the bike is ridden long enough to find out. Before that, everything is a guess. Some educated guesses, some not.

If you son doesn't ride much yet then if you aren't already, you might consider getting one of the lower tier models of the Checkpoint that are less expensive. Then if the size, geometry, fit and other things aren't quite right getting a more correct bike will be easier on the wallet.