probably it possible in "detailed" style only
If you mean the recursive size of the directory (its contents), this is not done due to performance impacts, as SpaceFM does not use a cached vfs. You can see this size by selecting right-click+Properties.
If you're determined, you can also write an event handler that displays the recursive size of selected items in the status bar. My Burn Tools plugin contains a working example of this, but this is not currently a built-in feature.
like it did in totalcommander, after pressing spacebutton showing directory size in Type column and step cursor to one position down. It useful if need to know and compare sizes of few dir's.
Dir sizes are now calculated in the 'bg' branch. For alpha testing or comments please use #582.
Works well here, even for cifs drives. I changed settings accordingly to see the directory sizes of the nas.