Recently, I downloaded an app called Size My Bike at the suggestion of my training coach. I have ordered a Trek 520 for some upcoming tours, and am making alot of component changes to suit what I want. The model that I took for a 50 km (about 32 miles) test ride was a 57 cm (23") frame and 175 cranks. I now have the frame and have made alterations so its mine. However the cranks have not been installed yet so there is the ability to change out if I decide to. I am 5'11", 33" inseam. The SMB app, suggests that I should be riding a 50 cm (19") frame with 165 cranks. Last evening, I was riding my mountain bike on my trainer, and when I am in an aero position, my legs are coming up to my chest, and actually rotating out at the top. I measured my crank length on the MTB and its 180. (yea, holy crap!) love the bike though. The 175 cranks on the test bike felt comfortable to me, but I didn't have the bike on the trainer, nor did I have aero bars it when I took it for a test ride. My concern is that when I build up the new Trek, that my knees might be rotating out at the top or hitting my chest or I won't be able to fully get into an aero position because the cranks are too long at 175. The difference between 165 and 175 is about an half inch. Factor in the the 180 crank to a 165, that's 3/4". The SMB app was pretty well bang on with the MTB except for that my cranks are 180-everything else was spot on or within millimeters. Coach's experience was that it was spot on with his bike(s) and he paid $300 for custom bike fittings. LBS did a basic bike set up for me to test ride the 520. Sorry to be so long hopefully I have provided enough info. Thank you for your thoughts.