This is the button: Calculate Folder Sizes.dcf (383 Bytes)
See: How to use buttons and scripts from this forum
In general, if you've just turned off the default Menu or Operations toolbar and need something from it, you can probably just turn it on temporarily to get things out of it.
If the toolbar has been modified (or imported from an old version), it can be reset to the current factory settings via the menu you get when you right-click an empty space on the toolbar, but that obviously loses any of your own modifications.
If you want to use modified versions of the default toolbars, it's a good idea to copy them and edit the copies, so you can always open the default ones if you want to get things from them. That's what I do myself.