Strelitzia "bird of paradise"

Dr Hessayon says repot in spring - also that they won't flower unless they are constricted! So I think if it were mine, I'd probably wait. JI 3 is the advised compost.

I think you could put the pot into a larger pot with loose soil in it so the exposed roots can stay alive until a proper repot in the Spring.

I assume that if you just had it delivered that this is a young plant? If that is the case you won't stop it flowering if you repot at this point. But next spring, when it is going to put on a spurt of growth is the best time. As Poaannua says you could 'double pot it if you are worried and as Spritzhenry says JI 3 is recommended because it likes reasonable drainage. Mind you I have put them in bog-standard compost from the supermarket before with no obvious ill effects.