how to change font size of lots of text boxes

Hi @lizrackster:

That is what paragraph styles are for. You create a new paragraph style, define it and assign it to multiple paragraphs. When you edit the definition and it is InDesign's job—not yours—to locate and update each occurrence of that paragraph style. 

Alternatively, you can use Shift to select multiple frames on a single spread, and then enter the new type size in the Character panel. You will need to work through the file spread by spread. After doing this once, I suspect you will understand why paragraph styles are so important. 

~Barb