Answer: The Doublefile Viburnum 'Mariesii' can reach 10-12 feet in height and width so it sounds as though yours have just about maxed out. It's always best to do light pruning every year or two than to radically prune back mature shrubs, but you can do it if you do it in steps. You can safely take off one-third of the live plant material in any one pruning session. If your shrubs need to be reduced more than that, plan to do it over a 2-3 year period, removing one-third this year, then reducing it by one-third again next year. By the third year, you can do some additional pruning for shape. Now is a good time to prune your shrubs back. This will give them all summer to recover and to produce new growth. Next spring, take them back a little further and they'll fill in a bit more.
Best wishes with your viburnums!
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