Height and size doesn't matter. As long as conformation doesn't pose any health issues in general, then they shouldn't be a problem in endurance. However, what does matter is metabolics and especially attitude. You're taking a risk with a younger, unstarted horse because you don't know how she'll handle the distance physically or mentally. Given that she's an Arab, she'll more than likely be physically fine. However, many Arabs just aren't up to it mentally and shut down after so many miles. Personally, I wouldn't take the risk with this one if I was planning on doing endurance and rather opt for a 4 or 5 year old that's shown some potential for really loving the sport. If they don't love it, it'll be miserable for the both of you.
Another issue - Arabs don't mature until very late. I wouldn't put one under saddle until 3, and they're not allowed to compete in LD's until 4 and endurance until 5. However, for an Arab that is still growing, you're much better off starting them in distance riding when they're 6 or 7, so you'll have to wait quite a while if that's what you want to do with her. Especially in endurance, if you take it too fast at all, it could be detrimental to the horse.