I like the ability to override the fonts during compilation, but would it be possible to optionally change the size of the font as well as the font itself?
The standard override formatting feature (particularly with the Override font only option is enabled) is a better tool for the job if you want to do more than simply change the font family.
The quick override is meant for cases where you do want to retain the font size, but apply a universal font family to all text, be they chapter titles, sub-headings, footnotes or page footers.
Yes, a tricky one.
The problem is that I have a number of documents at the same level that require a different font. The book contains a series of memos; some are hand-written, some are typed.
Okay, I think I’m asking for the wrong thing here.
The problem is that I used the ‘compile as is’ on these documents, but the screen font is difficult to see, but if I set the view size then the text for the other documents is too large.
Mmm.
Zoom doesn’t change the size of the printed font. It magnifies the screen, leaving the underlying font settings exactly as they are.
How does it make the text too large if the text wraps to the screen? 
It’s a personal thing. Just a matter of finding the right size so I can read it comfortably.
Okay, I see better what you are working with. Quick Font Override wouldn’t work well for what you want then, since you mention needing different font families to depict different writing sources. I would second the suggestion to use the editor zoom feature, to make it easier to work with these sections that must be formatted to output. They should be using the correct print size, if they are As-Is, with the display size of the text magnified dynamically using the zoom tool, in the footer bar, far left side.