Hi Steph,
I just left a comment and then decided to delete because I think I gave you the wrong information.
The CSS stylesheet you need to perhaps alter is MyMobiles/style/media.css
Here is a link to the master copy for Moodle 2.2.4
As you will see there are a number of font sizes which may be related to the screen you are talkinh about.
If you have access to your Moodle site files you just need to change the value of the font-size to whatever size you think will work best.
Hope this helps?
Mary