Unfortunately the font-face and size of the auto-complete text presented in the edited drop-down as you type-and-match is hard-wired.
I've seen two strategies for changing the auto-complete text:
1. handle auto-completion in code, and change the ComboBox SelectedIndex as you type.
There's an old example of that here on CodeProject: [
^].
Another example, virtually identical code (copied from CP ?), from 2012: [
^]
What you lose with this approach ... even if you modify the code to show the DropDown ... is seeing in the DropDown only the items that are potential matches. And, if you do modify the code to show the DropDown, you may find your mouse-behavior kind-of strange, since ... I hypothesize ... the DropDown "captures" the mouse.
2. create a custom control that shows a ListBox instead of the default DropDown: lots of work, that. You can find some examples on StackOverFlow, like: [
^].
Conclusion: I suggest you avoid trying to do this unless absolutely necessary.