The length shouldn’t be an issue, the width is tight but you are riding small horses so it might not be as much of an issue as it would be for someone riding a big horse with a long stride. If you are only doing walk, trot and a little canter, seems you should be okay. Ideally, a wider arena would be nice, 65’-70’ best but if there is a lot involved and it’s costly, I would live with what your have.
I used to board at a barn with a 60’ x 160’ indoor, and that was pretty tight, especially with more then one rider using it. Plus, one end of the arena had a 10’ long rise, so coming around one short side, you were riding up hill, then down hill a stride later. So, yes, try to extend as much as you can.
