The header and footer fields on the appearance maintenance page is a bit restrictive in size but does allow HTML tags. The prologue and epilogue fields also allow HTML tags and are also less restrictive.
Here are some examples of how you can set the text larger in those fields:
Surround the text with a header (h) HTML tag. These tags go from H1 (largest) to H6 (smallest)
The header and footer are limited to 128 characters so it can make fitting more complex text in there a little difficult. The prologue and epilogue on the other hand allow large amounts of custom text.
Hope this helps. Please let me know if you need any additional help.