It's CTRL+SHIFT+F that brings up the standalone font dialog. You can then tab to the size combo box, or hit ALT+S to shoot yourself straight to it. Once you're in it you can either type the point size you want or use up/down arrow to move among the combo box's dropdown list values.
Of course, you have to have selected the text you want to change the font size (or any other font setting) for first.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Explanations exist; they have existed for all time; there is always a well-known solution to every human problem — neat, plausible, and wrong.
~ H.L. Mencken, AKA The Sage of Baltimore