PlantLife Online

Answer

When transplanting woody plants you need to take into consideration the age, size, growth rate and species of plant in question. The Gardenia you're dealing with is generally a small, slow growing shrub and your case it has been in the ground for 2 years, I don't expect the roots have gone too far. I often advise people to take the largest rootball they feel they can safely move intact relative to the size of the plant (If a plants is 24" tall and 24" wide don't take a 48" rootball, "Relative to the size of the plant"). A general rule for moving trees and shrub around the garden goes like this; for every inch in dimeter of the trunk take 7-8 inches of rootball in all directions (e.g., A 1 inch trunk would have a rootball 14-16 inches in dimeter). Soil weighs a lot! Most roots grow within the top few inches of the soil so depth is less important that width.

Since I don't know the size of your gardenia you are looking to move I can't give you a definitive answer about the size of pot to place them in during the transplant. When done the right time of year and given the right care after transplanting (Planted in the Garden Landscape or in a Pot) most trees and shrubs can withstand an amazing amount of root loss. Water! Water! Water! Do not let the rootball dry out. 

The links below are a few other questions about the topic of relocating plants in the garden. 

http://plantlifeonline.net/questions/transplanting-shrubs-trees

http://plantlifeonline.net/questions/how-transplant-my-maple-tree

http://plantlifeonline.net/questions/replanting

Hope this helps.