When it comes to mixing your water, the last thing you want is any type of difficulty. It needs to be large enough to hold a good amount of saltwater in, but not too large so you have difficulty moving it to the tank.
It needs to be easy to add your salt to and you need to be able to physically mix the water (I use a large plastic stirring spoon). You also need to be able to place a heater in to warm the water.
As with the jerry cans above, these mixed water storage containers also need to be safe for foodstuffs. This way you know they are safe for your aquarium.
I use 7.8 gallon buckets which are not that difficult to haul around and my preference has always been to make up only 5.2 gallons of salt mixed water at a time.
When working out how many buckets you need to purchase, remember that you will need: