It isn't possible. MusicBee only gets tag values from individual tracks and doesn't really have any concept of calculating values across an entire album.
Exceptions are places like the status bar, where MusicBee has a preset output format.
There are other threads about this. I'll see if I can find one.
-Edit- I just noticed there is a calculated
Album Duration tag. But not one for album size.
So it might be an easy addition if you were to ask nicely on the Wishlist forum.
-Edit edit-I found the recent thread I was thinking of, but it doesn't include any more information than I've given here already.
I know I've made a post fully explaining this topic in the past with proper pictures 'n all, but damned if I can find it, and 2020 being what it is I can't remember if it was 6 months or 2 years ago.
Bee excellent to each other...