Hmmm...
Go to Admin CP -> Styles & Templates -> Search In Templates
Choose to search only in this style's templates...
Search for:
If nothing comes up look for:
(without the space)
One of those should find a CSS template with that font value (and hopefully only 1)
Edit that number in whatever template, it should be able to reduce the font size.
Hopefully it will do it without screwing up the search box.
The ultimate issue is this custom style is poorly designed to have these classes effecting your search box- may be nothing that can be done without rewriting part of the style.