Hi Angela
You have understood correctly. One of the limitations of a 3D printer is definitely the size of the model that it can build. However, as you have said, this limitation can be overcome by cutting the model in the modelling software in to parts with flat surfaces so that they can be glued together at a later stage.
To join the parts there many options available as of today and most of them work equally well with ABS and PLA. One can use epoxy adhesives or cyanoacrylate adhesives (Superglue). A chemical solvent can also be used for bonding. A solvent generally melts plastic on the both the surfaces to be joined, fusing the parts together.
All the above mentioned ways are for plastic parts. In case of metal parts they can always be welded as per the material used.