I reccomend 1 unit = 1 meter because physics uses the metric system
... which is why it took Newton so long to 'discover' gravity, because the English were working in feet/pounds.
You can choose any scale you like and 'physics' will still work - it's just that you wont be able to lift formulae directly from textbooks, other people's code, etc.
You may find that tweaking your own value of gravity will give better aesthetic results than slavishly trying to scale everything at 1u = 1m and using 9.8 m/s/s
Given the variety of tools used for modelling, and the fact that many use different base grid-sizes for layout, the idea of standardising may be a non-starter.