Alright Ill play around with it later and see what I can do. Will update Flexile Thread if I'm able to fix it for others with similar issues. Thanks guys!
Didn't have time to work on this until now. The flexile theme is using the "Node List" font size inside Style Properties: Forum / Node List. While I understand each theme is different, is there a specific category that handles the text size and css for "Show sub-forums popup"
I've been able to fix the issue but it also updates all the node titles and just trying to edit the size of the sub forum nodes that appear under the primary node.