I wouldn't personally put yoyo's in anything less than a 3ft long by 1ft wide and high tank (the bigger the better though), they are very active fish and to really look after them properly you need to keep them in a group of at least 5 or more preferably.
They prefer the cooler tropical temps and do best in well filtered tanks with sand substrate and lots of hiding places;
To add to what has been posted it is suggested that to keep them happy you need to have fish other then the Yoyos in with them. I do not have a problem with this seeing as I have a 55g with assorted Rainbows and others in addition to my 3 yoyos but they do seem overall quite happy so maybe it is valid.