For the enclosure size, get a large. Jackson can be very active chams - he will order all the space.
I have been using the Repashy LoD and NoD together, alternating about every other feeding. My cham is a Parson's, also a montane species. It's a system that is working really well. I know some experienced keepers use the LoD with nearly every feeding. Get the smallest size jar - a little goes a long way.
Looking forward to photos!