Gardenias do just fine in large pots -- when they do fine at all (see the suicidal gardenia thread and other tales of woe)!

Go one size up, so about 12" is right. You want room for the roots to grow, but not so much room that you get soggy soil at the bottom of the pot, which leads to root rot, which leads to death.

Try not to disturb the roots too much when repotting; just ease out the rootball and seat it into the new potting mix inside the new pot, then fill in, water, step back, and cross your fingers!

Congrats, by the way, on your success with your 'denia so far!

Jim