Based on measurements that were taken yesterday, and a trusty bike fit calculator, it would seem that I will fit best on a 55 cm 2015 Leader 735 frame.

I'm only 5'7" ish, though... 55 seems large. I know that it's impossible for you to recommend a proper bike fit over the internet, but in keeping with what you know about Leader frames, would a person who's 5'7"-5'8" be dwarfed on a 55?