I think (not positive) that some cartoonists will create block shapes and give them a unit size making it easier to scale the actual objects.
For example a tree which is generally 30-40 foot high you may set to 10 square blocks high and a building that is 13 to 15 feet high you may give it a single square block.
It may not be all that accurate, but it would get you in the ballpark where you could scale it more later when more objects are in the scene. It always helps to compare two objects when deciding the scale.
If you wanted to be accurate, you could google the actual size of the object then use a ratio to scale it. Then all objects would be to scale.