I like small, underpowered word processors for typing up quick software documentation or notes. TextEdit is the closest at hand on Macs, but it doesn’t support setting margin sizes. Luckily, it DOES preserve them if you set them manually by editing the RTF file with trusty ol’ emacs.
The settings you want to edit/add for margins are as such (from The_RTF_Cookbook):
\marglN – set left page-margin to N twips (default: 1800)
\margrN – set right page-margin to N twips (default: 1800)
\margtN – set top page-margin to N twips (default: 1440)
\margbN – set top page-margin to N twips (default: 1440)
Where a twip is defined as:
A twip is a twentieth of a point, i.e., a 1440th of an inch.