5'x5' and 5'x10' units are the most popular for students
The typical contents of a dorm room—desk, chair, TV, bookcase, twin bed and a few boxes—should all easily fit within a 5'x5' storage unit, which is the size of a small closet. As the smallest standard storage unit size, 5x5s are also the most economical, and are easily affordable for most college budgets. While a smaller storage locker (typically less than 2'x3') might be even cheaper, be warned that these units usually feature low ceilings, preventing you from taking advantage of the vertical height of about 8-to-10 feet that 5x5 units usually include. When packing your 5x5 unit, try to use same-sized boxes to make stacking easier.
If you have a particularly large bed or desk, or perhaps an extra piece of furniture such as a couch or futon, you may want to upgrade to a 5'x10' unit. Additionally, 5x10s allow enough space for another student's belongings—consider storing with a roommate or friend and splitting costs to save money.
Storage prices are highly seasonal—especially in college towns
Storage prices spike in the late spring and summer as moving season comes into full swing. This is particularly true in college towns, where you won't be the only student looking for a storage unit. April and May are likely to be the most expensive months, as students move out of their dorms during this period. Starting your search early and locking in a rate for the entire summer is probably your best option for mitigating this seasonal spike.
Consider Climate Control
If your campus is located in an area that gets hot and humid during the summer—which, of course, is almost everywhere—you'll probably want to rent a climate-controlled storage unit. Many of the things you keep in your dorm, such as books, wooden furniture, electronics, bedding and clothing are susceptible to mold and mildew, which proliferates when the humidity is high. Climate-controlled units are typically housed in a single air-conditioned building and thus not directly accessible from outside, which makes it harder for bugs and other pests to get at your stuff. Though climate-controlled storage units are a bit more expensive than the standard variety, your next roommate will probably appreciate that your things don't reek of mildew.
Skip the extra amenities and services
While some amenities (like climate control) are important to consider, there are others that will only add unnecessary expenses. One of the most common is 24-hour access, which you shouldn't need if you're heading home for the whole summer. Additional services like moving or shipping might add a bit of convenience, but are more expensive than a storage unit alone and may not be worth the cost.
Look for student discounts
It's not uncommon for storage facilities to offer student discounts, particularly when they're located near a campus or in a college town. Look for their advertisements each spring. Note that some discounts don't apply unless you meet the minimum tenancy length requirements, which may mean renting the unit for longer than you actually need. Be wary of "first month free" and "$1 move-in" specials that come with length-of-stay conditions.