An eventer can be any size. My cousin has a 15hh saddlebred mare that will jump almost anything. You should be posting conformation pictures. An eventer is usually built a little uphill, nice strong pasterns, nicely angled from hock to hip, and a nice engine (butt). The shoulder shouldn't be overly steep or overly sloping. Gamey helps :)
I would say (this is my own opinion) that you want a taller horse that is brave and that is willing to do anything and that can handle anything. Depending on what type of rider you are you might want to get a bit of green one because the spunk will help you a lot while eventing. I hope I could help you and good luck!