Unfortunately, "Total Size" is NOT what you are looking for. It shows the size of everything in the current folder including all subfolders, not the size of each individual folder. I wasn't able to spot anything that does what you want in File Explorer.
I'm going to dig a little more, if I get lucky and find anything then I will post back.
EDIT: After re-reading your post, I now doubt myself. If I have misunderstood and you want the size of the CURRENT folder that you are in with everything included in that folder, then
@KevTech is spot on. If you are looking for an entry that shows the size of each individual folder, that is what I have not been able to identify.