What did you buy? Cultivars vary massively in final height from quite small (4ft) up to tree proportions. Feral or generic B. davidii will grow quite tall unless properly pruned. Bushiness is also a function of the cultivar.
My advice is to reduce the height in October by a third and no more; this will prevent wind rock and encourage the dormant buds lower on the plant to develop. In spring next year, early April is best, cut the plant down to 40cm, say knee height, from the ground. Don't worry if all the green is gone as it should sprout within a week or two. Feed with a compound fertiliser (Vitax Q4) plus extra potash for the flowers. Then it only has 3 months to grow before flowering so it can not get quite so tall. Unless it is a rangy type you should get a bushier plant.
Do NOT cut right back in winter as it will die if we have another soggy one. It needs the leaves to breathe.