If you're trying to find a storage unit and can afford only a smaller size than you really want, you know you need to pack efficiently. But that goes not only for the boxes you have, but also the storage unit space. A few simple strategies will help you make the most of whatever space you can find.
Higher and Higher
Probably the single most important modification you can make to your storage unit is to add shelves. You don't have to install shelves on the walls; instead, get four- to five-shelf plastic or metal units, available at most home-related stores. Line the sides and back of the unit with the shelves; leave a few inches between the units and the walls so that air can circulate around the shelving units and boxes.
Shelves serve a couple of purposes. One is that they give you a nice, vertical storage space that keeps boxes off the floor of the unit. The other is that they prevent the boxes from crushing each other. That allows you to store more boxes in a single vertical column than you would be able to if you were merely stacking boxes on top of one another. The weight of the boxes would end up placing too much pressure on the boxes on the bottom of the heap. Using shelves eliminates that pressure. As a result, you end up with more boxes placed off to the side, freeing up space in the center of the unit for furniture.
Another strategy is to pack some items in smaller-than-usual boxes. You'll have plenty of those typical "small" boxes that you can get from moving supply stores, but find a few smaller and flatter ones through friends and places like grocery stores, which might have old, spare boxes you can use. These smaller boxes can fill in extra space on the shelves.
If you will be storing any furniture that you could potentially disassemble, do it. Returning furniture to a flat-packed state -- or at least a partially flattened state -- allows you to slip pieces of furniture between shelf units and other items.
When you check out the smaller storage space, talk to the storage facility staff about rules regarding shelving and see if they offer any shelf supplies that are reasonably priced. You can check with local facilities,such as Centre Avenue Self-Storage, to see what unit sizes are available. Even if you add only one shelf unit, or if you have only a few smaller boxes, you'll still be storing at least a few things in much more efficient ways.