Ok, limited knowledge of fruit trees here we come.
The size and planting spacing of you trees will depend mainly on a) type of fruit you want to grow, and b) the rootstock of the trees.
The species and types of fruit will determine, generally, how many trees you need. Some will happily self polinate, some need two trees of the same species, some two of different species and I think some may even need three trees for maximum yields. Most trees which need others to polinate are best planted within 90 m of each other, to ensure effective pollination (and the bees will help with this, definitely). Also important is when the trees are likely to flower. There's no point in getting two trees to pollinate each other, and then have one flower at the beginning of the season, and the other at the end.
Root stock is important, because that will determine the eventual size of the tree. If you're expecting a big, 5-6 m tall apple tree, and buy a dwarfing root stock, nothing in the world will make it grow over 2 m (for example). Root stock can also determine how long it will take for the trees to bear fruit, and how long it will take to reach maximum yields. To complicate matters further, each different fruit has it's own root-stock classification system.
Your best bet is to buy a book. Personally (but I don't have an orchard), I've found the "encyclopedia of gardening" which is publish by the Royal Horticultural society invaluable. It's a general gardening book, but has an excellent chapter on fruit (and one on veg too) with lots of pictures, and it explains root-stocks, flowering times and pruning really well. It might be a bit pricey - I don't know. I've had my copy for about ten years, maybe more, and think I got it from a "book club" for £10. It may be expensive, but I still hold it's worth every penny, especially if you're planning something on this scale.
God, I can waffle when I put my mind to it.