Jennifer wrote:

We get so little rain here, in the spring/summer it can go 3 - 4 months
without a drop. I was looking at the 40 - 50 gallon size.

What you're looking at is about 1 liter a day, you don't need any
stinkin' barrel. I pee more than that, I'm sure you do too. It's not
worth the trouble and the mosquitoes for so little water over so long
a period.

Where does your downspout water go otherwise? With so little rain
then the only real benefit you can possibly derive is to plant
whatever it is you want to water in the area about the terminus of
your downspouts. I get a heck of a lot more rain than you, my
downspout water is directed to my lawn and nearby trees... my
downspouts enter a buried 4" plastic pipe that runs below ground that
surfaces where the grade permits some 30' away. Planting something at
that spot then that's the only benefit you can hope to derive from so
little rainwater. In 3-4 months you'll lose more water from
evaporation than you can hope to derive over that time, in other words
if you save and ration you'll lose about all... you've come full
circle and are back to peeing on your coleus.