Adjusting the size of one-line text (label, title, subtitle) has always been an annoyance in Balsamiq. The only way to do it is to go in the size dropdown and select a new size.
I wish that at least, we had size-decrement and size-increment buttons, that would be much faster. But at best, since it’s not a paragraph, I should be able to resize text directly to what my eyeballs think, so having handles to resize and the size adjusting automatically.
Accessorily, the quick mockup feature (holding Y to drag a text area, then inserting text) should convert to the right size that it’s been drawn. Eg. if I dragged something that looks 72pt, then it should be a Title with 72pt, and not revert back to 24pt. And so on for other sizes.