The model can be any size, however the collision & existence (bounds) can only be 64. That means that any model over 64 can't have proper collision or brick grid.
Additionally, the engine seems to cull based on the bounds, so if a model extends the 64 size and the actual bounds are not in the player's field of view, the model will be culled, although visible if the actual brick is in the field of view. This is rather inconsistent as far as I seen though, so it might not be what causes it.