Cargo Van: You have a small 1 bedroom or studio apartments with little to no furniture. Larger king/queen mattresses often do not fit. Also, keep in mind that cargo vans often do not come with loading ramps.
10'-17' Truck: You have a 1 bedroom or studio apartment with an average amount of furniture. For trucks closer to 10', items will need to be vertically stacked, so if you have delicate furniture or very large pieces, you might consider getting a truck that's closer to 17' in the search results for this range.
17'-20' Truck: You are moving a 2 bedroom apartment or house. This is a popular size range for couples and small families with average sized furniture.
24'-26' Truck: You are moving a larger home, bikes, lots of furniture, and the job MUST be done in one shot. This is the largest truck you can drive without a commercial license.
Here's my advice: if you're teetering between two sizes, just get the bigger one. You're probably going to spend more in gas making the extra trip to go back for that little coffee table you just couldn't leave behind. That's my thinking.