Jayne: IMHO the best water tank is a concrete one, built on-site for the purpose. (the plastic ones not only use fossil-fuels in production, but leach PHPs and also have a tendency to burst.) best place to site it is under a deck - or indeed, as an integral part of a deck - or "buried" into a slope in the garden (then you can use the top for a BBQ/etc area). either way you dont take up valuable space and its visually unobstrusive.
as for cutting out gutter waste, a simple mesh filter, inline in the downpipe (and available as a pre-built section in any decent plumbing store), will divert all leaves etc from the intake.
capacity is the key; depending on average rainfall, and whether you have a long dry summer, allow around 1800 gallons capacity per person, perhaps as much as 2500 if it gets really dry for, say, 4-5 months. so a 6000 gal tank in a temperate climate will do for a 3-4 person household.
any questions, happy to advise.
Written in July 2008