The beginner will start drawing without first thinking about how big everything should be compared to everything else.

As a result, a house or a tree might end up looking only slightly bigger than a person.

This may be fine for a gag-manga, but in general, a pro will plan out the proportions of the elements in a panel before starting to draw it.

It's not necessary to think in terms of specific units; All you really need to think about is how big something is relative to something else.

So a character might have a size of 1.5, while a door may be 2.0, a house 5.0 and a tree 7.00.