I have a very experienced proven stallion now, but I have never trained a colt to breed
First you dont have to train a colt to breed instinct does it all :bgrin Sorry couldnt resist.
Me personally I would probably go for a proven stallion.
If you can see the baby with in the first few months then you have a good idea what it looks like. After that they start changeing so its hard to say. As for height, you can measure the cannon bone and times by 4 add 2 and that will give you a idea of how tall he should get.
With a proven stallion, you know and hopefully can see what he has produced. He know his height, and you know what he looks like. Not all stallions are bad, and I truelly beleive the bad ones are the ones that were not worked with or not worked with properly.
Sure we have had some stallions that are a bit of a handful and in a hury to go if they know mares are there. However none of them kick, bite or any of the other stuff people often think they do.
Of coarse we havent had one as mellow as Little John ever was though.
I defenatly would stay away from a yearling. This is the time for many changes and its really hard to say if that is what they will look like. Some go through a ugly phase. And some look nice when you see them but could completely change before they are mature.
one-two if their tail has hit the ground, then they are generally done growing up
There is no truth to that comment.