If you hover over a folder with the mouse to trigger its tooltip, that will also trigger calculation of the size of that folder (assuming the size info is in the tooltip, which it is by default).
You can also select the folder(s) and push Ctrl-K to calculate their sizes. (If none are selected, all folders below the current location are calculated.)
Or you can do it using folder formats, so that particular folders will have their sub-folder sizes calculated, while others won't.
Some more detail here: