Of course, the question in the title doesn't have a unique answer. It depends on what you have to pack and move. Lucky for you there's a whole range of moving box options available.
Even something as seemingly mundane as a medium packing box takes on special qualities when it's made by pros, for pros. Whether it's a 20" x 14" x 12" or 24" x 15" x 10" box (both approx 2 cubic feet), it's still going to be made to the same high standards as all professional packing and moving supplies.
Ditto the extra large moving boxes, such as 24" x 24" x 14" or the truly expansive 26" x 26" x 12". These boxes aren't simply bigger. They're made to withstand higher pressure (a measure the pros call ECT - Edge Crush Test, or the Mullen Burst Test). These boxes carry the highest ratings and will stand up under any normal, heavy-duty use.
Naturally, there's more to a professional moving box than simply space and durability, as important as those are. That space has to accommodate the specific types of items you need to securely and efficiently pack and move. You can't put a 27" tennis racket in a 20" box. That's why an entire range of special-purpose moving boxes are available that serve a purpose that is really pretty common: storing all your goods for moving.
Wardrobe moving boxes are the first special-purpose box that might come to mind for most people. These 24" x 24" x 40" boxes aren't simply longer; they're made just for clothing. No need to fold, pack, move, then unpack, unfold... and spend a week after your move ironing. You take them off the rack, hanger and all, and just place them into the box on the special-made metal bar.
You get that convenience without sacrificing peace of mind, you can bet. Not only are they the perfect size for your suits, dresses, robes, and other long clothing items, the box is secure and durable. The rod fits snugly into carefully crafted holes. The box itself has other holes: hand holes to make moving simple and foolproof.
Kitchen moving boxes are another specialty item that everyone will need. The designer of moving boxes knows that dishes are at special risk during relocation. Even careful professional movers can't guarantee a safe move if you don't use the right box for the right item. Boxes made especially for dishes fill that need and provide easy packing and peace of mind.
These boxes are designed to a rating of 44 ECT, making them super strong. The cardboard layers securely surround a highly pressure-resistant corrugated inner. Have you ever noticed how a bridge has arches? That form helps resist high downward pressure. You see that principle adapted in the best moving boxes when you examine the interior layer.
A similar principle is at work in Picture Frame boxes. Here's another instance when it's all important to be able to resist forces at the corner which are actually responsible for more and worse damage than pressure on the flat portion. At 26.5" x 6" x 41" they'll fit all but the largest prints or paintings (or mirrors).
There are many more moving box options available, each one just right for your packing and moving needs. No matter how much stuff you want to move - or what odd shaped possessions you have - you're likely to find suitable and affordable moving boxes to put them in.