I have never attempted anything that large but I would guess about 2 ft. around the base and at least that deep.That should get most of the roots.Are you putting it back in the gorund or going into a large container?Good luck with it.

Dig around the drip line (outermost edge of the leaves) preferably with a garden fork. Be prepared to water it well through the summer and give it some root stimulator. If you plant it in full sun, try to rig up some way to give it some shade in the hottest parts of the afternoon. You can also prune back about 1/3 of the plant to help you move it and to help prevent water loss thru the leaves.

Hello again,

I have transplanted the gardenia and will see how it goes. I dug around a wide area and have put it in a part sun but mostly shade place with excellent soil (lucky to have that in Georgia clay land!). Thanks for the advice...I will follow.