Try this:
From the Clipboard in the game, go to Options > Controls > Map Keys
Remap the `(KEY_TILDE) for message size toggle and ' (KEY_QUOTE) key as Check 6 in Global settings.
Note: You could try using another unused key (such as the / or * keys on the NUMPAD) if your keyboards are strange, but I don't like to advise anyone to change from default keys because there is no way to reset all to default if you make a mistake.