Hi Jon,
On the app tab, app content section there is the ability to add some of your own CSS (the language that controls things like text size).
On phones the default is 0.9em and tablets it is 1.5em.
If you add
body{font-size:1.5em;}
then it will be bigger on phones etc. You can experiment with that number, it’ll affect the menu size too.
If you want just to change the content text size then go with
#rendered .content{font-size:1.5em}
You’ll need to click the refresh logo at the bottom of the app home page to download fresh data if you have been on the app today already.