Always worked for me as Mathias said.
If you have something selected & hit <space> or use the folder+KB icon, the selection returns a size.
If you have nothing selected & hit <Shift+Alt+Return> everything returns its' size.
What happens when I select something & use <Shift+Alt+Return> is interesting though
EG:- in a dir with 7 folders & 15 files -
1 - select some files - 5 of 15 & hit <Shift+Alt+Return> = all 15 files are calculated
2 - select some folders - 3 of 7 & hit <Shift+Alt+Return> = 3 folders + all 15 files are calculated
The way MC differentiates between folders & files for a mixed selection in these cases of calculation is not something I noticed before as I always select something or nothing.
Nice find AlanJB