How to coordinate the sizes of objects in the game's open world?

Question

Game development

Let's say there is an open world, for example, let's take GTA VC (not the best example). As we know, VC has a modular world consisting of many objects, roads, traffic lights, trees, weapons, cars, etc. etc. How in such games, the sizes of objects are consistent with each other? Simply put, what practices do game developers use to keep objects proportioned?