If you're worried about temperature swings, the larger, the better.. Ultimately, yes if this is your concern, burrying them would be good too. But you have to remember, trees will root out until they find conditions that they DO like. So if the roots escape their boxes while burried in the ground they will keep going and going and going until they find favorable conditions. If you are looking for some reasonable confinement for the roots and burrying them, I would look into something called a root control bag, these allow you to burry the root ball, in soil of whatever composition you like AND keep the roots confined in the area you desire. Makes it much easier to dig later as well as prevents a lot of unnecessary stress to the tree when you do dig it back out.
I haven't ordered from these guys, but it's simply one example of the types of products that are available,
http://www.hort-enterprises.com/
Another thread here turned me on to these and I think I'm going to be using them extensively in the future..