In what file do I edit widget default font sizes?

I’ve done this before — I need to edit the file that will let me change the default font and head/title sizes for all my sidebar widgets. I found it last time by doing a very slow search of all files on my site, but lost the information as to which file that was. If someone could remind me, I’d appreciate it. It was a raw code php file fix, not an admin page fix or a theme editing fix.

I just discovered that my sidebar headers look enormous on a 1024 resolution screen. Joyjoy.