With 10' ceilings I might go to the next size up if they sell a 7.5" baseboard.

Agrred.....is mdf ok??

I don't understand why you think MDF doesn't hold up in climate change.

I have custom milled MDF trim on all my interior windows and doors. It is the most stable product out their if you want custom wide trim applications

Prime and paint as usual.

Speaking about humidity. Lyn, did you use in your bathroom?

We had MDF throughout our home in FL - high humidity area - and had no problems.

MDF is fine. there is no humidity in the desert! every home in so cal has MDF baseboards.

The 5 1/2 plain style base is fine.

yes you can use it in your bathroom. The molding is primed and painted. it will be fine.

MDF is very durable everywhere, except in bathrooms near the tub/shower where people aren't careful about not getting water on the floor. I have MDF throughout my house and in 20+ years the only problem has been right next to the tub in the guest bathroom where the MDF got damaged from water puddles due to sloppiness. I replaced it with finger-jointed wood with the same profile to avoid this problem in the future.

I've used a lot of MDF for trim, even in bathrooms. Prime all six sides with a quality oil based primer that can be sanded (some can't). If you make a cut, prime the cut end. Sand lightly to get rid of drips and brush marks. Then use a good acrylic paint as finish. You can even put one coat on prior to installing, then final coat after.