Build 484. Most current.
Preview (F8): When keying in new font size becomes “disoriented” and it is impossible to enter correct 2 digit character size if less than 40.
Entering 11 becomes 14, 12 becomes 24, 13 becomes 34, 14 becomes 44, 15 becomes 54, etc., up to 19 => 94
Entering 21 becomes 14, 22 becomes 24, 23 becomes 34, 24 becomes 44, 25 becomes 54. etc.
and so on.
Keying 40 on up seems to work correctly.
FC 7.5.0 is build 512. Are you using FC6.x? 
Glad you asked. When you request an update check you get downloaded and installed build 484. It teases you with “7.5.0.512”, but it delivers 484. Maybe I’m using pirate copy of FC, all these years!!
After update:
Check your shortcuts and installation folders. The update was obviously installed elsewhere.
What isn’t obvious is why Dick’s second screenshot not only shows ‘(build 484)’ but also shows ‘Version 7.5.1’. Where did the ‘5.1’ come from? Dolby Laboratories?? 
Initially, I just let the installer do it’s thing, but this time I checked each step. The installer put 512 into a Vn 7.0 directory. So I redirected it into the directory 7.5 and got 512! Now back to the testing … thanks to Both of you.
Ok. Running Vn 512! Problem still exists as noted at first.
Preview (F8): When keying in new font size becomes “disoriented” and it is impossible to enter correct 2 digit character size if less than 40.
Entering 11 becomes 14, 12 becomes 24, 13 becomes 34, 14 becomes 44, 15 becomes 54, etc., up to 19 => 94
Entering 21 becomes 14, 22 becomes 24, 23 becomes 34, 24 becomes 44, 25 becomes 54. etc.
and so on.
Keying 40 on up seems to work correctly.
I can confirm the (mis)behaviour. FC is trying to impose its minimum and maximum of 4pt and 512pt respectively, so if you try to type 12 it sees the 1 and replaces it with a 4, leaving the cursor to the left of that digit so that the 2 gets added on in front, giving you 24 (with the cursor in between the two digits).
In this particular example, you can put the cursor in front of the 2, type a 1 and then delete the 4. Alternatively, you can leave the cursor between the 2 and the 4, type any two digits to give you a four-digit number which FC will correct to 512, and then delete the 5.
We’ll try to fix this behavior with the next upcoming release.