This is a long, long running difference between the Windows and Macintosh environments. From the beginning, Macs used a resolution of 72 dpi, matching very closely the printing convention of 72 points per inch — yes, I know that before computers it was 72.27 points per inch … who came up with that one?
When Microsoft finally began to get a handle on windowing systems and “WYSIWYG” — Windows 3, I think — in their usual belief in “Bigger is better” they set the standard resolution as 96 dpi. A one inch box on Windows would be 96 x 96 pixels, on a Mac it would be 72 x 72.
So on a Windows box with the default setting, a 12 point line occupied 96 pixels vertically; on a Mac 72 pixels vertically, so using the same basic screen technology, the Windows one looks much bigger. Result … to many users who’ve always used Windows out of the box, 12 points looks too big for comfort on screen so they use 10 point. Mac users like me get a Windows document in 10 point and it looks too small … except Chinese Windows users tend to go for 10.5 pt, because of Chinese characters.
You, it seems, are used to and like 12 point at Windows pitch, so on a Mac it looks too small for you. But Scrivener to the rescue … set your editing font to say 14 point or even 16 point, and save the 12 point for when you compile.
Or you could switch to another font like Georgia, which definitely looks bigger to me, or you might like to try Gentium — SIL has just launched a new Gentium Plus, though some of the Mac specific features are some way off, apparently — which looks a little bigger, but is more open. And if you’re into wierd and wonderful languages, it is designed to handle non-Roman as well as Roman scripts within the single font, albeit not CKJ systems, I imagine. But complex diacritics and IPA, definitely.
Personally, I really dislike TNR … I have to compile to it, unless I’m producing a PDF, for the benefit of my colleagues who don’t know anything else exists … but for myself, I use Adobe Caslon Pro, which is smaller than TNR, but given my work and the periodic need for IPA, I’m thinking of giving Gentium Plus a try in my editing environment.
Mark