Hi Mate,
i'm 1,84m and i had mostly 19" bikes.
I was in the same situation like you. I bought a bike that is 17" and thought this will never work out but it is a very pleasent ride.
I gave this bike to a friend who is even taller than me and he loves it, too.
Maybe you find somebody with a small bike for a teast ride before you buy the Diamond Back.
For me it was no mistake to buy a small bike as a tall guy.
Check out my build thread of the Scott Superlite for some pics of a small bike and a big fella.
Cheers,
Adam