Snadger
Our plants have been in about 5 years and alongside some box and santolina/cotton lavendar make quite a good little ornamental hedge. They are pruned back quite hard on a regular basis so have never been allowed to get too big. They are no more than 9ins to 1ft wide. I'm no expert on this, but I would think you could trim them to what size you need. Seem to remember Bob Flowerdew suggesting raising new plants from cuttings from time to time to replace any plants that are getting too big or leggy, but ours haven't got to that point yet.
I hope you give it a go, Lavender seems to have so much going for it. The ice cream mentioned earlier sounds heavenly!