I'm 19 now, but I had about a years worth of driving under my belt before my parents even considered letting me pull a trailer. Even then, my Dad took me to an empty parking lot and gave me a "test". He made me drive through the city, around corners, and in the lot made me do lots of backing up exercises. The most difficult thing for me to get the hang of was backing up, it was only this year that I became Very comfortable with it. But, there are a lot of other things to consider, like if your tire blew would you be able to change it, calmly and quickly?
We have a small 2 horse slant and it was very convenient for me to learn with because it is so small. It gave me more confidence b/c it requires less space for backing up and if I am trying to find room in a parking lot I can maneuver more easily. With that said, I give my vote to a smaller trailer, you can always upgrade.
"Horses don’t have hard mouths, they have hard, stiff bodies. The softer you can get the horse through his body, the softer he will be in your hands." Clinton Anderson