I would not like two different heights to my baseboards. Either the 4.5 or 5.5 would work. Buy a length of each and put it up against your walls, in both the rooms with vaulted ceiling and the 8 ft ceilings.

Will your door trim be changed as well?If your door trim is old fashioned narrow, I would go with the 4.5., otherwise bigger is better, IMO.

@kathylawyer it will be the original door trim, a basic style probably two inches wide with a slight bevel inward. Good call on checking the door molding sizes and seeing which base board looks best. Thank you!

Here are 2 pictures, the first shows my remodel room with 5.5" door jam width a 6.5" floor board. The other from an older room is a 6.5 inch floor board and 2.5" door jam.... obviously I did not practice my thoughts to you...and now that I look at it, I would advise you to go with the 5.5 regardless of the door jam size.

I would go with the 5.5 inch throughout the house❣️When I had my 2 smaller bathrooms renovated, I went with the 5.5 inch. I love that look.

Depending on the order, baseboard is an 18th to a 20th of ceiling height. I'd go to a lumber yard that has a large selection of baseboard buy a few choices, put them in place and make a decision after a few days.

If you're planning to use all mdf base and casing, be sure the installers are sealing the bottom and back edges. One water spill, and it will swell up and never recover.

5.5 seems to be the favorite! I’m heading up to the house Saturday and will check out the height and depth again in all the rooms. Great tip on sealing!