I think you're right, small letter m is the largest single character that will be displayed. Note that letters can change vertical size when you mix them with others, for example capital letters added to lower case m will make the lower case m get shorter, since the upper case letter is now scaled to the vertical size of the text box. So there may be a way to change the displayed characters by bracketing them with spaces and another character on each side. It may be a way to fool the tool to get different font sizes.
For example you could simply enter <m> for one size
or
<M> <space> <space> <space> <m> <space> <space> <space> <M>
to get a slightly smaller m.
The idea is that the bracketed outside letters don't show up at all, and the bracketing keeps what you want centered. But I can't vouch that doing this won't reveal other bugs in the tool, or that things will remain centered vertically.
edit: I just tried it and it seems to work. You can also use the letter <p> on one end and <t> on the other end to get a smaller m that will also center it vertically in the box. And if you add different numbers of spaces on each side of the bracket you can also slightly shift the displayed letter slightly to the left or to the right.