Hi, I have Leylandi and Hebe. What I did find was that if I left them in the plant pots, growth would be restricted which has proved to be the case - although they still managed 1ft per year! Pruning is the answer as you do see many garden hedges under control. An unrestricted tree has grown to 7ft before I stopped its vertical growth. I have one very slow-growing one which is dark green and looks as though its frosted with icing sugar! This has managed about 1 inch per year. Sorry I don't know the names of these varieties - the only connection with green fingers is when I get green paint on them!
Mick