I'm 186cm and with a headset on it can just touch the roof, will have to strap in tight during turbulence! I also have the rudder pedals as far away from me as possible.
The best option for you would be to find a Diamond operator and pay them a visit - it will cost nothing to sit in the aircraft to see how you fit!
I should add I'm referring to the DA42 Twin Star - I haven't as yet been in a DA20 or DA40!