Best way of dealing with these to keep them in shape or to a certain size is to prune over annually in summer. As yours is so large, better to do it in stages, so take a third off this year and another third to a half next year. You can risk cutting it right down if you want, but it might not respond well to that.

I would suggest the 'one third per year' regime as well, starting this year as soon as all chance of frost in your area has passed. Make sure you give it a good general feed immediately after pruning and make sure it doesn't go short of water if we get a dry spell