How big is your butt? 😉
Personal question: How big is your butt? Water butt, of course 
My garden is about 15 metres long and 6 metres wide. Mainly grass, with about a 1 metre deep border on each side and at the rear.
I like the idea of a water butt, but it would be frustrating if it's too small to be useful.
So, how big is your butt, and how big is your garden? Do you find your butt is big enough?
How big do you think my butt should be? I'm on the edge of London so we don't get as much rainfall as most places. Here's a picture of my garden, in case this helps:

My garden is about 15 metres long and 6 metres wide. Mainly grass, with about a 1 metre deep border on each side and at the rear.
I like the idea of a water butt, but it would be frustrating if it's too small to be useful.
So, how big is your butt, and how big is your garden? Do you find your butt is big enough?
How big do you think my butt should be? I'm on the edge of London so we don't get as much rainfall as most places. Here's a picture of my garden, in case this helps:

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I have a 90 litre butt in my veg garden and it is entirely inadequate - I only use it to water the plants that grow inside the teeny polytunnel and I'll empty it in a week if it doesn't rain. I've just acquired a 240 litre one to go alongside it, which hopefully will improve matters. 3 weeks without rain is a rarity here.
Overall though we have another 700 litres of storage elsewhere in the garden, which gets used mostly at the moment for making concrete and washing cars, rather than watering the garden, but I can put a pump in one of those big barrels and pump water to my veg garden to refil the little one when I need to.
I would try to store as much as you can. Use it to water newly planted plants and anything in pots, but established plants in the ground you should not water as a rule, as it makes them more vulnerable to drought. Try to limit the number of pots you have to match the amount of water you can store, so you don't have to water with tap water.
The one by the terrace where I grow acers and hostas and mints in pots is half full now they've started needing watering.
Answer - get the biggest you can afford and preferably tall and slim so it can be discreet.
I have one 200L and another 100L.
Wisdom is not putting it in a fruit salad.