Resolutions used on models and acceptable sizes.

[Acies]

2048x2048: Very few. Used for characters, very large things(think buildings, landscapes - altough tiling techniques not available in HPL might be preferrable etc.) or an important center-piece in an area (let's say you're building a temple with a 'god-statue' in the middle which the player will be inspecting/be wow-ed from).
1024x1024: The size I use for most objects; walls, doors etc.
512x512: For smaller things of lesser importance; a pot, office supplies etc.
256x256: For the smallest things which I don't really care about, but have to be there to tell an environmental story; could be a pen in an office for example.

Basically:
1. The larger something is on the screen the greater the texture resolution it should have.
2. That, combined with how much you believe the object will be 'inspected' (will people look closely at the ominous grandfather clock?).

In case of using text (computer screen, paper etc.) which need to be readable that can be solved with higher priority on the uv map (larger island/UV) or a separate image of appropriate resolution.

Optimization-wise you should be fine sticking mainly to 1024x1024, but in case you are having a lot of different objects in one area it might be preferrable to go for 512x512 for those objects (garbage dump for example). It sort of falls under 'rule 2' as mentioned above; due to the massive amount of items the player probably won't go around and inspect every one of them, but rather take it in 'as a whole'.