An apple tree that won't get too big?

The size of an apple tree depends on the roots, not just the variety. The breeders have done some good work and bred several different rootstocks which give varying vigour. I did a Google on 'apple root stock' and it came up with www.mikesgarden.co.uk/appletrees.htm which probably gives you a good background on the subject. I would also advise searching for specialist fruit tree suppliers and read their websites too.
Apples are not self fertile - you need two with overlapping flowering times to get a fruit set. However, if a neighbour has an apple tree, this will be sufficient as long as they are both in flower at the same time.
As for plums, I did some research as I am planting one ths autumn. The one I finally chose is Czar - it is similar to Victoria (which is reckoned to have the best flavour) but has better disease resistance. Again there are different rootstocks (but not as many as for apples) which will determine the eventual size of the tree

AndrewR is dead right about apples. However, as for plums it depends what kind of plums you like. Czar is really for cooking, it has a strong rich flavour. If you like that kind, damsons are worth considering too, they don't need any care and produce tons of fruit. We also have a greengage, which is an early, sweet, green plum, lovely eaten fresh, but with a delicate flavour which does not really survive cooking. You need to research about pollinators or consult the supplier. We have a lot of trees and two damsons and I don't know if they are self-fertile.

My Victoria plum is on Pixie rootstock and not getting too tall although you do need to summer prune to keep a good shape. It is self pollinating but remember to ruthlessly thin fruits to one every 4inches or all you get is lots of stones covered in skin!

Me again. I also have a family apple tree with 3 different sorts on. they all pollinate each other and its on a dwarfing rootstock.

Hi there

Thanks for all the helpful info - will give me something to think about over the winter months

any variety you like can be budded on to a rootstock, by yourself or the nurseryman.IT IS THE ROOTSTOCK THAT DETERMINES THE SIZE OF THE MATURE TREE.

the stocks are classified as follows,

M27, most dwarfing, for stepover or dwarf cordons, slow growing, so good for compact spaces if summer pruned.

M9, semi-dwarfing, for oblique cordons or small bush tree, or centre leader/spindlebush. (ok for container gardening or in the ground)

M26, for a semi-vigorous tree, good for bush, or delayed open centre bush, or espalier. ( too big for container growth), will reach about ten foot tall and eight foot wide.

MM106, or M25, very big, think old fashioned orchards big bramley trees grown as standards whose crown is at six foot from the ground.

get your trees from Blackmoor nursey near Alton in Hampshire. It may have been budded by me.
Go for disease resistant cultivars, especially if you are an organicist.

best regards from james