Unless it has been UK grown and most likely to have been hardened off outside, then I would be inclined to put it in light, sheltered aspect until the spring. When you finally get to pot it up then a JI ericaceous compost will be best.

If you mean Pieris Forest Flame, that one's not too keen on direct sunlight, unlike Pieris japonica varieties, so if it is that, don't stand it in full sun. It prefers light shade or dappled shade.

Don't put it in a planter that is a lot bigger than the root ball though, unless you plant something else round it until it gets bigger. Its best to avoid having a lot of compost with nothing growing in it. If you really want a particular planter that is too big pot up the pieris in a smaller pot and sink it into compost in the bigger one. They you can pot it on when it grows without disturbing the roots much.