@sacha81
I agree that the defualt font is a little bit large.
I made the change the magicmirror/css/custom.js . And I just scaled the ".MMM-Trello" class. Hope this helps.

@wangxum168
Could you please share your custom.css for Trello? I'm trying to change that too with no luck.
Thank's un advanced,

To do a manual scale:
In MMM-Trello/MMM-Trello.css add the following at the bottom:

.MMM-Trello {
  transform: scale(0.7);
}

This will scale it by 0.7