Is there a standard scale for building "minifig scale" creations?
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.