For the difficulty curve I don't think you need to limit the lengths too much, even though a few short levels in the beginning grant easy successes for the player, but maybe introducing different kind of obstacles like moving walls and jumps gradually. For a rough example could be that there's a jump you need to make on hole 3, and a few different jumps on holes 4 and 5, then on the 6th there's a rotating wall but no jumps and only combining those two a bit later on the 8th hole or something like that. It doesn't necessarily have to be just a single mechanic or obstacle every time something is introduced, but it's good to have a safe level for learning how to fail and succeed with the new mechanic
What is the correct pacing of new challenges so that it doesn't get too boring or difficult is up to you and your level designer :)