Here is a rough/generic rootball size guide* for transplanting trees and shrubs. Soil type, tree species, and other factors are not taken into account, but it will give you an idea:
For trunk diameter below 1/2", multiply the diameter by 20 to get approximate rootball diameter.
For trunk diameter between 1/2" and 1", multiply the diameter by 18 to get approximate rootball diameter.
For trunk diameter between 1" and 1-1/2", multiply the diameter by 16 to get approximate rootball diameter.
For trunk diameter between 1-1/2" and 2-1/2", multiply the diameter by 14 to get approximate rootball diameter.
For trunk diameter between 2-1/2" and 4", multiply the diameter by 12 to get approximate rootball diameter.
Here are guidelines for rootball depth:
For a rootball with a diameter of 1', depth should be approximately 8".
For a rootball with a diameter of 2', depth should be approximately 1'.
For a rootball with a diameter of 3', depth should be approximately 15".
For a rootball with a diameter of 4', depth should be approximately 18".
*Also see ANSI Z60.1