I use spray bottles for my seedlings, then I have 1/2 gallon jugs and 1 gallon jugs... but I usually only grow in 3 gallon pots so I don't need a lot more than that. Other people will mix a 5 gallon container and then dip into it to water their plants.... Whatever works for you as long as you know the exact amount of water you're using and mix in an appropriate amount of nutes!
Good luck