You can prune them annually, but whether you would be able to keep it at a particular size is questionable. I think give it a go and you should at least get a few more years out of it, particularly if you look after it a bit.
This is going to sound counter-intuitive because I know you want to restrict growth, but if you make sure it gets water over the summer and a winter mulch around its roots, it will do better. You will get more new growth and it will be denser, which means an annual prune will leave it looking neater. This is how plant nurseries make topiary specimens - treat them well, but prune with determination.
It may go bedraggled and bare in the centre eventually, but if it does, it does.
