If you choose font size 9 in the preferences for "history detail composition" (and other detail composition views), you will be able to use this size in history window. But if you change the text size in this window to 10, and once again want to re-size it to 9, this size is no longer available.
And if you did not set this size in your preferences, it would not be available in history window.
I am using ACT! Premium V16.2, and checked it with Arial, and some other fonts.