Collapsible water carrier sizes are measured by the gallons / liters of water they can carry. Some units have 5 gallon capacities. Others can hold 2.5 gallons, 4 gallons and so on.
These units are designed to carry water. These are most appealing for campers and hikers. Many have an integrated handle so they are easy to use. The high-end carriers are of BPA-free construction and leak proof.
One of the most important features is the molded-in handle so you can manage the pouring. Durability is also important. Many of these carriers have a top neck so pouring is less difficult.
The collapsible water carrier sizes aside, the material used is crucial. It should be sturdy so it can be used in different environments. Second, the material must be flexible. Some carriers can fit inside the average milk crate. This makes storage straightforward.
Since the carrier is collapsible, you can pack it when no longer in use. But flexibility should not compromise its structure. Some collapsible carriers are structurally not sound. You have to avoid these. Remember that some of these units are not stable. If you store them in the back of your car, they will tumble when you cross rugged terrain.
This is absolutely essential. Camping sites are rugged and accidental bumps cannot be avoided. You want a good unit that can withstand rugged environments and settings. These are more expensive but they are worth it. Some poorly made carriers will leak when filled to the brim.
Even the best carriers are not going to last if you don’t take care of them. As much as possible, avoid dropping it on the ground or bumping it hard against rocks. Carriers can last a long time if they are taken care of.
You have to determine how much water you need. Many like to overestimate their needs so they don’t run out of water when camping. This is generally a good idea. However, you should avoid overly heavy carriers. They will be hard to carry.
Many people buy extra units. For example, you can bring one at camp, another for home use and another for cleaning chores. Note that some of these do not have handles. These can be difficult to hold.
Ultimately it is not just the collapsible water carrier sizes that matter. You have to look for units that pack easily. They must also be lightweight.