Magnolia's by nature are a neat lollypop shape in general. Without seeing the tree in question it's not easy to recomend what would be best. If it's growing to one side then by all means trim it back to a balanced shape. If it is just too big for the area then a general even all round cut. My Mags already have their flower buds showing ready for next Spring. I would prune after flowering also the dryer weather helps the tree wounds to heal quicker with less chance of infection which is possible in the wet weather.