I just thought of a much better way to do this which works with a single button:
Go to Preferences - Layout - Lister Styles and create a new style called "Flat Dir Sizes" or whatever you want.
(If you have the Styles toolbar displayed you can right-click it and do "Add Style" to do the above step quicker.)
In the style, set it to use Flat View and then turn on the Format checkbox and edit the format.
In the format, Filters tab, turn on the Hide Filter for Filename = *, and then in the Columns tab add a column such as Total File count which will force the folder sizes to be calculated.
(In theory you shouldn't need that column and could, instead, change the Get folder sizes setting in the Options tab of the format. In reality, Styles seem to ignore this flag. I'll file a bug report on that, but adding the column instead seems to do the trick.)
You may want to define another style which does the reverse of this so you can get back to normal easly.
Now you can either use the Styles toolbar to jump into this mode or you can make a button which loads the style by name:
Prefs STYLE="Flat Dir Sizes"If automatic size calculation is turned off in Preferences and Folder Options, and none of the columns which show sub-item counts are displayed, then GetSizes will only happen each time you run the GetSizes command. There's nothing to turn off as such.