How do you find your bike size in 29er?

i have been riding 16" mtb with 26" wheel. i would like to switch to 29er. how would i find the size for 29er. how high will be small size. I am going to buy ORBEA 29er H50. the problem is i cannot try the bike size myself. i am getting them through courier. P.S. i have never ridden 29er.