majority of all your small to mid size sets will fit in 6x6x4 8x6x4 12x9x4 12x12x4 14x8x4 15x15x4 16x12x4
your mid to large sets fit in the following
20x12x4 20x16x4 20x15x6 24x16x4 24x16x6 24x20x4 24x20x6 and 24x20x8
now as you can see on the larger ones you can go with the bigger and eliminate some but if you get an opposite coaster you will pay for it dearly in the dim weight. Like 2 or 3 of those could almost pay for your pack of boxes of the smaller size.
those sizes pretty much knock out every LEGO configuration though. The 15x15x4 isn't used that much and that one you could just go with the 20x16x4 as there is no dim difference on that.
Same with the 20x16x4, that could do the 20x12x4 which is a common one. Only advantage to have both in that case is if the smaller keeps it under another pound which saves you $1-3 per package depending on zone.
There's a few others like 12x6x3 that fits the pullback technic sets and keeps them under a pound so they can go first class. That can be a $2-6 savings right there if going to farthest zone from the difference of 16oz and 2lb priority.
So it is really important to study rates, dim, zones as well and know what you are shipping and volume. A lot of people make comments about box costs but having the right box in a situation can save and pay for many other boxes.