If you have Vertrees/Gregory, Japanese Maples there is a section on pruning to control size.

Here is a link that might be useful: Tree Heights

There are lots of selections that will naturally stay within your requirements, even more so when grown as container plants. Yes, the constricted root space and the contrived environment of container culture serves to restrain and slow growth - think bonsai on a slightly grander scale :-) A few that come to mind immediately are Kamagata, Mikawa Yatsubusa, Coonarra Pygmy, Pixie, but there are many more that work equally as well. And a good many have been cultivated from sports specifically for bonsai culture. I find many of the dwarfer forms are actually better served as containerized plants, as they are easily "lost" or overpowered by larger, faster growing plants in the landscape unless placed in conspicuous isolation.

I an personally not a huge fan of pruning much of anything to control size, preferring to let plants assume their natural height and habit and only pruning to enhance that habit and improve form and maintain health. I find this to be even more relevent with Japanese maples, but that is a very personal opinion.

My personal recommendations for dwarf acer palmatums that stay aroung 5 ft tall/wide would be:
Mikawa yatsubusa, shishio hime, kamagata, baby lace, koto ito komachi, or red pygmy.

The following could reach 10 ft in 20+ years, but will stay much smaller if kept in containers:
shaina, shin deshojo, tsuma gaki, aka shigitatsu sawa, beni maiko, or fjellheim (a coral bark dwarf).

Many of the dissectums can reach 15 or 20 ft in 20+, but again if kept in a container they could easily be kept below 10 ft I think:
Inaba shidare, filigree, waterfall, or crimson queen are just a few of the hundreds of cultivars.

Keep in mind that many of these dwarfs are finnicky, and may not survive in poor environments.

I agree with GG with the exception of weeping dissectums which can be pretty easily trimed w/o butchering or distorting them as is NOT the case with many upright palmatum,Amoenum, Scolopendrifolium,Matsumurae etc which are hard to really keep small if they are large full sized varieties...David

thanks. with regards to 'poor environments', what is a good environment. i live south of london, and want to have one in a north facing garden. exactly how much sun is best? do they need shelter from wind and rain?

I'm bad with environments outside of the southeastern US, but I can tell you that you need to be careful about the following:

hardiness - someone else take this one because I've only had experience with heat/humidity being a problem

wind - particularly for 'shin deshojo', 'aka shigitatsu sawa', 'baby lace', and maybe 'beni maiko' (I think the others should be OK)

Rain shouldn't be a problem as long as there is GOOD DRAINAGE. You don't want root rot a couple years down the road.

The cultivars listed under wind probably need part shade, as full sun could hurt them.
I think the others should be ok with full sun or part shade, but the colors are more outstanding in sun.
I think morning sun until noonish followed by dappled shade during the afternoon is ideal.

Like I said I'm bad with environments but I think the above holds true more or less anywhere.