You can change the font size in Preferences > Interface > UI Font Size.
Are you talking about the UI font size, or the font size you're using to type things into documents?
If the latter, just select the T tool, and without a document open change the font, etc. to your preference. Then quit/close Photoshop. Next time you open it you should be using that font/size.
If that's not working for you, something may be wrong with the saving of your preferences files.