Oops, I read this question before I saw a similar one just below it. If the ones you bought are in plastic bags follow the instructions for that one... Below is my original reply.

It depends what they are in now. Its better to keep potting on than to put them into a pot thats a lot too big. Choose pots that are about three inches more across than the present ones and be prepared to move them on when they have grown. If you want to use the eventual decorative pots straight away you can sink ones the correct size into the eventual ones and fill in the gap with more compost which you can then plant up with small annuals . Check each year that the roots have not spread through the drainage holes into the surrounding compost or the trees will be too hard to remove to pot on without damaging the roots. Use a John Innes compost rather than a peat based one. For the final potting on JI 3 would probably be best.

If they are bare root then a pot that gives 3" either side of the roots when they are spread out.
further down the questions there is another about growing them is pots. Have a look at this too.

Thanks, I will go shopping for a couple of nice pots once I know how large the roots are.

Don't put them in their final pots yet because they will grow a lot bigger. See my suggestion above.