Is there a way to make it overall smaller?

Make use of "<size=15>Text here</size>" in the "Chat format" configuration option.

I have the following already but it didn't change anything:

"Chat Format": "<size=13><color=#989898>{message}</color></size>",

Did you reload the plugin after editing the config file?

Yep...

Merged post

Nevermind it worked, silly me. Thank you!