Answer: The general advise for pruning trees and shrubs is to remove no more than one-third of the live plant material in any one pruning session. If your magnolia is quite overgrown and you want to reduce the size, you may have to do it over a 2-3 year period to avoid stressing the tree. With that in mind, you can reshape your tree - just don't plan to do a major make-over in a single growing season.
Best wishes with your landscape!
Q&A Library Searching Tips