What limits the Twinmotion in terms of scale of the project? What would be a best workflow for efficiency in model handling?

The limit is linked to the power of your graphics card. We have invested in high power PCs with quality gaming level graphic cards to run our most complex projects. If that is not an option, I suggest checking the quality settings, it may be worth dropping to the lowest setting while working to ensure that you can save the file. If you still have issues, try to reduce where you use super detailed features like trees and grasses, or turn off complex model pieces that are not visible in the rendered views. Also, common mistake is using lights with Shadows turned on, this can kill a project if your computer can not handle it.