The world is a huge Cartesian space.
Boundaries[]
The physics engine uses JavaScript Numbers, which are float64. The limit of float64 is between -1.797693134862316e+308 and 1.797693134862316e+308 but levels using such high values don't load.
Sky4ce09 experimentally figured out that values from roughly x: 5*10^17 to x: 6*10^18 represent some sort of x-axis limit (example being 5.7976931348623157e+17). However, weird things happen at such distances, the aim line disappears and collisions no longer work.
Sizes[]
For reference, a Hoppi measures 40.0 distance units and a Door measures 100.0×160.0 distance units.