5x5 is probably a reasonable estimate for size at 10 years in the ground. I have seen Nova Zemblas that are at least 8 feet tall, but have no information about their age. In general, no rhododendrons, even the dwarf varieties unless they grow in a prostrate manner really "top out". Only pruning will keep them at specific height.

The size depends on the amount of shade. Ours are in full sun on the south side of a stone wall (our house) and they are easy to keep at about 5' tall. The can't go any further than 5' wide since there are other rhododendrons on either side and they have grown together. They are in front of windows, so we don't let them get more than 5' tall. I doesn't take too much work. The easiest way to prune a mature rhododendron is to pinch back new growth on the top right after they bloom.

In shade or semishade they will tend to grow more open and taller. But as maingrower mentions, you can easily keep them back to 5' tall. It will make them fill in if they are too open.

I have a Rhody Nova Zembla that is around 15 feet tall and just as wide. Anyone have an idea on how I can trim it down several feet without killing it

Since it is so tall, I would guess it is in a shady location. Be cautious about pruning too much at one time. Rhododendrons will quickly come back when pruned severely if they are healthy and in a fairly sunny location.

To safely bring it back, prune no more than 1/3 each spring. If you prune before the end of June, you should be OK. Pruning in the summer will remove next years flower buds.

You must prune just above growth joints. Each year as the plant starts to grow there is a visible point where the plant started growth. We call this point a growth joint. Prune just above this point, because that is where the dormant growth buds are located. Don't prune between joints, because there are no dormant growth buds in that area. If you do prune in the wrong place, no problem. Just cut off the dead stub that is left when a bud further back develops and the extra length of stem dies.

Here is a link that might be useful: How to care for rhododendrons

"Maturity" often means a mere 10 years. That's how someone can easily have one 15' tall. An old place can have rhododendrons that have been there 50 or more years. Plantings here have hybrid rhododendrons over 20' tall.