Hello I have just bought a small magnolia is it ok to plant out now or better to leave in pot thanks

That would depend on the location and more! Can you supply more details..ie; location - windy, fertile, sandy, exposed, etc?

Generally, if the plant looks too big for the pot then it is time to move it to a more suitable location. You may find some general information on Wikipedias Magnolia page - Magnolia - Wikipedia, the free encyclopedia