I live in Derbyshire, UK and have a Magnolia tree which in recent years seems to have gone a bit out of control, i.e. much bigger than I think it ought to be for the space it occupies.
I feel it needs to be half to two-thirds its current size, but am unsure if such brutal pruning would harm it. Also, when is the best time of year to do so? I have included a picture of how it currently looks.
You can do some pruning now, its not quite too late as its early winter. However, heavy pruning all in one go is not advisable - deciduous magnolias can take a long time to recover from pruning, and hard pruning is likely to cause a lot of watershoots.
If you want to reduce its size, the best way is to do it over 3 years, reducing the top branches and twigs by no more than a third, and preferably only by a quarter each time, but even then, you may find watershoots increase and the tree may not flower very well for quite a while. Further info under Pruning and Training here https://www.rhs.org.uk/plants/popular/magnolia/growing-guide
