I love my fleece Cuddl Duds shirt (it happens to be a small, which fits well, but maybe I already stretched it out? Small sounds...small). I could defintely get lots of wear out of new Cuddl Duds fleece shirts, size either S or M. And since it's usually a base layer, the color and/or pattern aren't important to me.
Another favorite is my old (as in, falling apart) L.L. Bean silk long underwear. I didn't realize they were so pricey, and I don't even remember how I got this pair in the first place. I'm sure someone else has some that are comparable but less expensive. I wear a size M in bottoms too.
While I haven't been outside much this season, I do have a job as a crossing guard once we're back in school in person, so I will need to be ready to dress warmer. Luckily my sizes don't tend to change much over the years, so hopefully I can get what I need as soon as possible, even if I don't have to wear it for a while yet.
What's your best keep warm tip? In my home office, I finally have a space heater this year. It ~ is ~ heavenly. I actually drag it down the hall when I go to hang out with my son doing his remote learning in his room. I also greatly appreciate the heated mattress pad on our bed. Each side has it's own adjustments, so I can turn it up to high before I get in, while hubs keeps his side a little cooler.