The roots don't stay small, even if you prune the top growth. Be aware of that when you plant them, and keep them well away from any buildings.

But would the roots spread as they would as trees?
I have both in my garden and have experience with others and have not found the roots to spread too far. One well established beech hedge was easy to remove, the hornbeam I've never tried but when planting around them I've not found any big root systems. Isn't there something about the roots of trees spreading as far as the canopy? Would a plant kept back as a hedge need to spread its roots out if it was getting all the nutrient it needed nearby?
I'm not clear on the answers to these questions but I've not had any trouble, so far.

I'm with Spritz, the more that you trim the hedge the bigger and more vigorous the root system becomes.

Normally, consistent trimming will slow down the growth of the root system, but the roots will still eventually get quite large.

If its not to late,the way to contain trees height is to plant them in a root bag.

Thanks for your comments everyone.I'm going to find out about the root bags thankyou Jcee.Sounds like a good option.

Good luck with it. :-)