The only way to adjust the font is to either have the system use a completely different font that appears smaller, or re-theme the entire OS. Neither is going to be satisfying to you, the latter quite difficult.
I've tried switching my System font to Fira, Lucida Grande, Input and Source Sans. All the fonts look good, most are larger than San Francisco but some are smaller. All have issues displaying in some form or another.
So the short answer is no, there is no app that will do what you want.