In message , KK
writes
Hello, - there have been several posts about water butts recently.

How big does it need to be

Depends on how much you want to consume. In general I'd say the
bigger/more of them the better.

& what if it overflows ?

Are they easy to plumb in to the downpipe ?

If you use the special diverters you can get for downpipes, then once it
is full it doesn't fill up anymore and the water just goes straight down
the downpipe.
--
Chris French