If you go to
www.keepers-nursery.co.uk and search on rootstocks, they have a page dedicated to the different root stocks and their characteristics. Basically most fruit trees don't grow on their own roots. The top will be the part chosen for the quality of its fruit e.g. Cox's apple but it's grafted onto a different set of roots. The root stock determines how big the tree will grow and can also affect some disease resistance. If you have poor soil, you can assume they won't grow as big as on good quality soil so you need a more vigorous rootstock because one that is intended to turn out small trees anyway may not cope at all in poor soil.
The other complexity is how you intend to prune your trees - the form - and that has books dedicated to it. Many commercial orchards now just have trees trained with one central leader and few/no side branches (e.g. Ballerina). This means they can plant them together which in turn maximises yield. I prefer the traditional orchard look so I'm taking out the central leader at a certain height and will train them with a balance of side branches. Depending on the height of the trunk you allow, this can be a bush (short trunk of 60-90cm, tree total height about 2.5-3m), a half-standard (trunk of about 1.2m, total tree height about 3.5-4m) or a standard (trunk of about 1.8m, total tree height of 5m plus). There are loads of other shapes too including the highly trained forms such as espalier, fan, step-over etc. but you ideally need to decide before you choose your root stock how you want your orchard to look. If you choose a dwarfing root stock, you'll never get a happy standard tree out of it - it'll just look sad and spindly. Equally if you want small bushes, you should never choose a very vigorous root-stock otherwise it'll just rebel and end up throwing out shoots everywhere.
Another useful source of info is Brogdale - the national fruit tree collection and full of lovely people! They've got a lot of helpful info on
www.brogdaleonline.co.uk including growing guides which will have pictures of the different tree forms amongst other things.
H