I was going to post this video! Lol
But to answer OPs question, it really depends on you and your horse's comfort level, as well as your horse's athletic ability. If you horse is most comfortable cantering around 3' jumps, you would not be wise to try and jump 3'6" from a trot. I have jumped my pony who is 13.3 and very athletic 3' from a trot. She may have been able to go higher, because she could jump 3'6"-4' at the canter, but I was not comfortable trotting jumps any higher than that.
For your horse, I'd say start trotting jumps at 2'6" and raise them gradually to see where you are both most comfortable. I'd start with a vertical too.