Two units of the same floor area can hold wildly different amounts. Height, door width, and the walkway you keep decide it.
Everybody shops for storage in square feet, because that is the number on the price list. It is also the number that tells you the least. A 10 by 10 and a 5 by 20 both give you a hundred square feet, and in practice they hold entirely different amounts of furniture.
Our units are 8 ft 6 in to the ceiling. That is a third more usable volume than most people plan for, and it is free. Stack boxes to head height along the back wall and a 5 by 10 will swallow most of a one bedroom flat, which is why we so often talk customers down a size at the counter.
The unit you rent is measured in square feet. The unit you use is measured in cubic feet, and the difference is roughly a third.
A four foot roller door will take a sofa turned on its end, a standard wardrobe, and a mattress. It will not take a full size dining table without the legs off. Before you book, measure the largest single thing you own rather than adding up the volume of everything.

If you will visit monthly, leave a corridor down the middle and rent one size up. If this is deep storage that you will open twice a year, pack it solid to the roof and save the money. Deciding which of the two you are is the single most useful thing you can do before booking.
None of this is complicated, but no price list explains it, which is why we drew every unit to scale on this site instead. Look at the plan, look at your largest item, and pick the honest size.
No deposit, no card at reservation, and the rate you see is the rate you sign. Cancel free up to the day you move in.
1400 Northgate Way, Oakland
88 Ferry Landing, Alameda
27 Gilman Street, Berkeley
Three storage facilities across the East Bay, run by the people who built them. Clean units, honest rates, and a gate that works at six in the morning.