Although we do not know the full details of Heechee physiology, but from the size of the spacecraft that you see before you and the number of seats in them, we know that they are of similar stature. The Ones, Threes and Fives are so named because they can carry respective number of passengers - but don't expect a generous bunk for your trip - we are talking about space travel here and even the Heechee with their technology have to economize fuel.
Hey! You there DON'T touch anything I haven't told you to. You don't want to be responsible for any damage I can tell you.