Changing view size rather than print size?

For some reason unknown to me, when I set my text to 12pt, which is a comfortable printing size, it appears in the document window at what looks like about 6pt to me. I have tried zooming, but that seems to either flip between full screen or half screen or change the font size so it’s comfortable to view but prints at 24pt.

What obvious thing am I missing?

I don’t think you’re missing anything. A 12 point font - especially Times New Roman, for instance - does look rather small in most OS X programs at 100%. You can either scale up the view (using the % pop-up button in the footer bar) to view at 110% or 125%, or use a bigger font and have it changed back to 12-point when printing (using the Compile Draft “Formatting” settings for printing the entire draft, or the settings in File > Page Layout > Settings > Scrivener > Text Options for printing a single document).

Aha! Brilliant. Am now using the % thing.