I could be wrong, and you may get a better answer, but to my knowledge I don’t think there’s a good way to get the exact size of a character. Mainly due to the fact the size can vary depending on the font size, anchors, text box size, and with certain settings (Size to Content).
You’re probably better off setting the dimenstions, spamming keys until it fills up the box and then counting the number of characters that fit. Make sure to set “Auto Wrap Text” to true to make sure all of the text doesn’t stay on one line.