As @jmc302005 said, this simply isn't possible, and that's intentional.
Try this, it's an excellent way to get an overview of what's eating up you're space
WinDirStat - Windows Directory Statistics
Just run and it and look for the big blocks in the treemap, then go clean them up if you can and run it again to see the impact it's had, simple :)
I think the reason the file size column exists is because the data is available on individual files, so a folder with files and folders in it would need the column there for the individual files.
As a result it would be a waste of time (and processing power) to program the function to not provide the column if there happens to be no files in the folder, only more folders. Especially considering the fact that you could add or remove files at any time.