The correct shortcuts are Ctrl+] to increase and Ctrl+[ to decrease.
Ctrl+} (that is, Ctrl+Shift+]) and Ctrl+{ (Ctrl+Shift+[) are not assigned to this function by default, at least not on US keyboards.
Similar commands that increase/decrease all selected text to the next size listed in the Font dropdown are Ctrl+> and Ctrl+<.
The first two commands, as Stefan mentioned, are called GrowFontOnePoint and ShrinkFontOnePoint, as opposed to just GrowFont and ShrinkFont for the other two.
The difference between these commands can be seen easily in the range between 10 and 14. If you use Ctrl+> or Ctrl+< on 12-point text, you will get 11-point and 14-point, respectively. If you use Ctrl+] or Ctrl+[, you will get 11-point or 13-point.