Magnolias don't like being pruned - you should only do this if strictly necessary, maybe to keep your tree in its allotted space or to cut out dead branches. It should be done in mid-summer as the 'wounds' take time to heal, and certainly now is the wrong time.

If you do have to reduce its size, choose just a few of the tallest or widest-spreading branches to cut back - but NOT until next summer!

...and then thin out the 'water shoots' that will sprout all around the cut. Otherwise you'll get a broomstick effect which will spoil the shape.