Congratulation to your wonderful early spring blooming tree. If it is a Southern Magnolia it can become huge. If it is a tulip magnolia you are still looking at a 20' tall and wide tree. After your Magnolia has finished flowering, you can then let it prune back. Although they don't like to be pruned heavily.
Trees and Shrubs to Prune in Late Spring/Summer, After Bloom:
•Azalea (Rhododendron species)
•Beautybush (Kolkwitzia amabilis)
•Bridal Wreath Spirea (Spirea x vanhouttei)
•Flowering Crabapple (Malus species and cultivars)
•Forsythia (forsythia x intermedia)
•Hawthorn (Crataegus species and cultivars)
•Hydrangea, Bigleaf (Hydrangea macrophylla)
•Lilac (Syringa vulgaris)
•M A G N O L I A (Magnolia species and cultivars)
•Mockorange (Philadelphus coronarius)
•Mountain Laurel (Kalmia latifolia)
•Rhododendron (Rhododendron species)
•Serviceberry (Amelanchier x grandiflora)
•Slender Deutzia (deutzia gracilis)
•Weigela (Weigela florida)
Trees and Shrubs to Prune in Early Spring, While Dormant
•Bradford Pear (Pyrus calleryana)
•Butterfly Bush (Buddleia Davidii)
•Crape Myrtle (Lagerstroemia indica)
•Flowering Dogwood (Cornus florida)
•Flowering Plum (Prunus blireana)
•Glossy Abelia (Abelia x grandiflora)
•Golden Rain Tree (Koelreuteria paniculata)
•Honeysuckle (Lonicera fragrantissiam)
•Hydrangea, Peegee (Hydrangea paniculata ‘Grandiflora’)
•Potentilla (Potentilla fruticosa)
•Redbud (Cercis canadensis)
•Spirea (except Bridal Wreath) (Spirea japonica))
•Wisteria (Wistera species)