Select the text and then using the Paint brush icon. You can either change the size of the selected text, or choose one of the existing styles. Body is the normal choice.
Note: Even without selecting text, changing the style will normally change the font/size/ect for whatever paragraph the cursor is in.
Here is the online Pages User Guide:
Pages User Guide for iPad
And here is the free iBooks version:
‎Pages User Guide for iPad
There are way too many options for me to go through them all here. But, when choosing a large project it is generally better to choose or adjust the styles to ones you like and then use them consistently. If you randomly change text and font’s through the document things can quickly become unmanageable when you need to make changes later.
Also, the auto table of contents feature uses the Title styles to generate the table. So a little planning can help a lot in the end.
I would make a small trial book and play around until I was confident I had what I wanted before tackling the larger project. Probably should have said that when I mentioned the possibility of using Pages for books earlier.