To the original poster:

The smaller the tree is when you plant it, the better it will establish and grow on. Most potted trees in garden centres are pot-bound - their roots are going round in circles in too small a container. You can tease out the roots to a degree but it's never quite the same.

If the tree is bare-rooted, it has lost a good proportion of its roots when removed from the soil. The smaller the tree, the less proportion it has lost. Commercial forestry nurseries, for example, undercut their seedlings in situ to encourage new growth of feeding roots at the expense of support roots. This means the seedling will feed itself well when planted out but it has to be small enough not to be blown over.

It is rare to get a really good tree at the normal garden centre. It's usually best to hunt out a specialist in the trees you are after.

Hope this helps.