The height needs to compliment the height of the vanity so it all ties in. If you make the beadboard 48" you can run it across the top of the vanity as a backsplash which can look very cool. If you go in the 30" range I would make sure it is below the vanity counter so you have a place to cleanly die it into.
I'm sure it is going to be fabulous!

Hi,
This will no doubt be a lovely bathroom when you've renovated it. Here's an ideabook for you that may give you a good idea of the beadboard height you want to go with. Follow the link under the picture:
https://www.houzz.com/magazine/renovation-detail-beadboard-wainscoting-stsetivw-vs~2448718

Wow...thank you all for commenting so quickly. >SB,LLC: Good logic! My specific concern about a 42+" wainscoting was the bend in the north wall behind the tub; that's one realllllly long mitre! I can back cut the upper piece so it's flush, which I think is the best approach. Possibly there could be a small transition strip to cover where both board meet in a butt. I'll post up the results when we're finished (hopefully) in July.

I thinking running a moulding at the butt is a great idea and I would definitely carry it around the whole room so you keep that look. Just be sure to line up the beadboard as close as possible. Look forward to seeing the end results!