I hope I can explain in such a way that it becomes clear.

Above the two keys marked with [ and ] you have two keys one of which is marked with a - at the bottom and an underscore above. And the other is marked with a = above which is the + sign.
Cmd and + is achieved by pressing the Cmd, Shift and =/+ keys simultaneously. That is what I meant; if you don't press the shift key (thus doing only Cmd = the computer beeps at you).

I didn't even think of using the numeric keypad (it works but it isn't very handy: you need two hands). I don't want to use the numeric keypad.

If Cmd = (I mean the key marked with = and +) is allowed (as it is in Preview: try it) you can get by with using one hand only.

I hope this clarifies what I meant.

So what I would like is that not only Cmd + is accepted but also Cmd = for increasing the fontsize.