Q: In TextEdit, how are margin sizes adjusted?

You can't adjust margins in TectEdit directly but, If you have a copy of Microsoft Word or NeoOffice (free), you can create a document with the margins you like and, then, save it as an RTF file and open it in TextEdit. TextEdit will use the margins created in the other program. If you want to create other TextEdit documents with the same margins, save the RTF file as a template.

I've been trying other coding methods for 30mins, gave up and found your comment - sorted in under 5mins. VERY helpful thanks!

TextEdit -> File -> Page Setup -> Paper Size -> Manage Custom Sizes...