you probably need 19inchs on a hybrid frame if not 18 go to the shop and sit/stand over some
I have a 21inch boardman mx race and it's almost to big for me 6"3.5 ,I think my legs are 34-36 and I barely stand about an inch over the frame.
I'd probably have been okay with a 19" which I think was the next size down but I was worried about my toes clipping the front wheel when turning.
loads of people comment that my bike looks huge lol
also wish I bought a road bike or a crossbike like everyone else who ever bought a hybrid.
So from what you're saying, would the triban 3 51cm would be alright for me? I've roughly 43 miles to the nearest Decathlon store so...