Most people choose an A5 (the examples on the website) or an A6 size card but everything is possible. Just email you wishes to email@example.com.
A6: 105 x 148 mm
US: 210 x 100 mm
A5: 148 x 210 mm
Straight or round corners?
Your choice but if you choose a thicker paper, round corners are not possible. So not an option if you print digital on Multilayer paper, on cardboard, letterpress or foilprinted cards.