Hi OP!
Do you use ElvUI? I ask this b/c I know it makes your font really small and can be annoying to try to resize.
(Just a quick note, I no longer have ElvUI, was far too much for what I want done in game, but I wanted to check with you first to see, so as to know what direction to go from that point.)
There is an addon that is lightweight called Extra Chat Font Sizes. The creator stopped work on it, but it does go up through BFA content. You might want to give that a shot and see if that works. I know it’s a temp solution. since the creator has stopped working on it.
I also just did a google search and in a past forum thread there were a few options posted that might help you out as well…I will link it below.
Good luck OP!!