The only times I have ever had to worry about margin size have been when I typed using a typewriter, and in high school computer literacy class when we did funky stuff with the very dated document editor. Otherwise, margins have always been pre-defined: lined paper for school and preset margins in office suite software.

So for unlined paper, and hand-written letters and such, what's the going rule? One inch was the rule with manual typing, but I've never seen it defined for hand written documents. The margins seem wider than on lined paper.

Make the margins no bigger than the reader's thumbs.

And no smaller than a Last Theorem!

FWIW

I think it is anything you want unless you are submitting to someone else who has a specific requirement.

For my own, I generally leave about a half inch handwritten and an inch type written. That is just my preference, YMMV.

I leave about ¾ inch margin on handwritten letters. I am not aware of any rules on margin etiquette... but I bet there are some somewhere.