A couple of questions that I find helps with size:
For example, take a hill. It's there for a few things
1) to slow your progress or divert you;
2) to block line of sight;
3) give you a better line of sight from the top;
4) to give the team on top a combat bonus.
For a compressed ground scale, I want the hill high enough to avoid LOS arguments, especially at 28mm, so a couple of inches tall at least. Less important for skirmishes, where it may just represent a bump in the landscape.
Generally speaking, if I want hills to be a hindrance, I still don't want a unit to spend the entire game struggling over it, so maybe not more than a couple of unit moves wide.
If I'm going to expect combat up and down hills, I generally don't want units slipping around or minis falling over. So I like stepped hills rather than rounded - fewer arguments about who's higher up, and no minis sliding down what's effectively a sandpaper surface.
So for a Warhammer game, I'd want a hill a couple of inches tall on each level, with a good 10cmx10cm (4x4 figures) space to stand on each level.
Houses are a good point - I'd say space to fit a unit inside, doors and windows marked for firing/combat points, maybe multiple floors based on your ruleset or needs. I found out in, a recent game when I thoughtlessly used 2-storey models, that you can squeeze double the firing capacity into a space if you allow more than one floor... and watched half my army disintegrate.
If you're intending lots of man-to-man combat inside structures I'd consider an exaggerated model or a separate space at the side of the table - for easier access if nothing else.
People who build in quiet times generally don't worry about protection too much; people who have experienced raids definitely think about interlocking fields of fire between houses, keeping the stuck roof on, wall thickness, angled or upper-floor doorways, ditches to channel attackers into killing zones.