I have 2 from Wickes, capacity 200 litres each, i.e. circa 40 gallons apiece. One is fed from roof of the garden shed (7 ft x 7 ft.),the other from conservatory roof,
12ft x 8ft.
Hope this helps.
Fred - I got a rebate on my water rates (only £13 but every little helps) because I could demonstrate that roof water goes into our two butts and not straight down the drain into the street