Really depends on the size of the plant you intend to buy, and how long it is likely to be kept in a container.

Magnolia Stellatas do grow very big! I'd put it in as big a pot as you can get and plant it out as soon as you can, you don't want to 'strangle' it.

Best to use a pot that's a bit bigger than the one you bought it in to start with, and then pot up into larger pots as it grows, say every year or couple of years. It will need a large pot fairly soon, say 18 inches deep by 12 inches wide minimum.

Make sure the pot is made of a suitable substance to withstand frosts etc...
... and welcome to GoY :o)

I would use a relatively inexpensive pot, too. Magnolias dislike root disturbance, and the best way to get it out of an old pot is to break the pot!

Thanks, all of you, for your replies. I ordered the plant online (on offer from JParkers). It arrived this morning; it's only about 10 inches tall. It would look sad if I put it in a large pot :). I think I'm going to get an inexpensive small-medium pot as Tugbrethil said and pot it on every year or so as Bamboo said.
I'm loving GoY. Can't believe I have been gardening( or trying to) for the last 4 years and registered here only a couple of weeks ago.

welcome to GoY naturelover, i know the plant you have as i got the same thing from jparkers a few weeks ago, mines in a 4 inch terracotta pot and will re-pot as needed.