What is the maximum size of model what you can use?

Max size is nominally 50x50x50m. This is the proper limit of the Roadway and Geometry LODs for to function. Anything larger may not work properly. i.e. the roadway may have holes in it which let you fall through. The same goes for the geometry. It may not prevent you from ghosting through walls etc. In some cases the absolute limit has been closer to 60m but to ensure proper functions its better to stick to 50m.

All geometry should ideally be made up from primitives with closed a convex shapes. eg A cube etc. (O2 does have a tool for the checking if the components are closed and convex).

While the limit is quite strict in the horizontal direction because of data structures used, it is quite possible one could create a lot taller objects.