It mostly comes down to common sense and some testing really.
Make sure you start high and see if it can go lower as the other way round is more difficult.
To have an idea on the sizes/relations i'll use my cabin map.
As this was my first ever try to make any VR or game map at all i have to admit optimisations were not high on the list. Luckily Hammer allows for fast updating on assets which i am still doing right now.

I used multiple textures for each asset (color,gloss,normal,refl,ao etc) as you probably know.
My terrain is an 8k map as i found 4k to be too blurry up close. I could probably play with UV density in certain area's and make that a 4k map that looks as good.
Bigger structures like the cabin consists of multiple materials each having 1k to 2k or 4k maps as i played with the resolutions to have a nice overal fidelity.
All other props are between 256-512 or 1024px keeping that as low as possible. Again where i had an object with scratches or text i had to go a bit higher. Most of the props are 512px.

Tileable textures have the advantage of keeping those textures really small against the obvious tradoff of having repetition. Some tileable textures are awesome like your tiles in concrete which work really well. My more organic environment did not allow much tiling as repetition becomes apparent quite quickly.

Hammer allows texturing layers that you can paint over eachother which is ideal for creating variation and keeping textures really small.

Thanks! That gives me a good starting point.