I experienced the same issue.
For some titles, the font is not fully adjustable.
My workaround, although I think Keynote trick seems good, is to combining titles.
I wanted to use Standard title with small sized font in top line center, with normal sized font text in bottom line center and tiny sized font in lower third center.
Instead of Standard, I used Centered title template.
There, the font is fully adjustable.
Then I added Lower Third title template.
N.B. both share same background
Adjusted font in Lower Third title.
Finally, I stretch out the text (purple box) to cover the both the Centered and Lower Third title templates.
The net result, title has text in Centered format with an apparent fade-in of the lower third text so ends with all three lines together.
