When moving it is often a stressful experience. The last thing you want is to have boxes collapsing and packaging adding to the problems. Double walled boxes are fundamental, Do not use single walled boxes as they will collapse when items are stacked on them. Also, try to buy the right size boxes, and modular so they stack easily.
Ideally a box of 18" x 12" x 12" is known as a book box in the trade. It is as it might suggest designed in mind of books and similar weighted objests......fill a big box to heavy and you won't be able to carry it.
A larger box might be in the region of 18" x 18" x 20". Generally a box of this size might be used for carrying kitchen applicances; like the kettle or toaster etc. Something more bulky and not to heavy.
Overall however, don't under estimate how many boxes you will need. Its amazing how quickly a box fills up when you start to empty a room that you thought had next to nothing in.