Is there a standard scale for building "minifig scale" creations? I've seen many impressive models built to "minifig scale" (e.g. the enormous USS Harry S. Truman aircraft carrier), but is there a standard (or at least widely adopted) scale e.g. 1:60 that these models obey?

It seems any scale based on the height of a minifig won't work due to the fact the minifigs are not well proportioned for people. For example, I stand taller than my car (as do most people) yet trying to build a LEGO car that is less than four bricks high would be extremely difficult and result in an unusual looking model.

Are there any tips on choosing a scale that allows well proportioned looking models, that doesn't dwarf the LEGO minifigures that ride in/on them? Is "minifig scale" dependent on the type of model (vehicle/castle) or generally deemed to be whatever looks about right?
It's usually 1:40, based on the minifig size.

However, keep in mind that a minifig doesn't have the same proportions as a full-size human being, so scaling from real-world to minifig-world is not an exact science. Just have a look at LEGO cars, you'll see that they don't really match real cars.