The reason we typically bump up the font in an RTF is that the Reader's RTF engine shrinks it down -- 16 points in an RTF is roughly equivalent to 11 points in an LRF file, and probably a bit less on paper.
How many pages a "normal A4 RTF" would be on the Reader rather depends on how many pages the "normal A4 RTF" has in it to begin with.
The margins can be set in the RTF when you save it, so if you set, say 0.05" margins before you save the RTF, then that's what you'll get on the Reader.
let me suggest you download and install Sony's PC software, then you can load an RTF in it, and get an idea of how much text appears on a page. The eBook Library software works as an emulator for the Reader pages, the Fonts will appear way (way) bigger on the PC screen, but the amount of text per page is spot on.
You can find a link to the eBook Library software
here.