When moving trees, we start to dig a circular trench the height and width of the spade around the tree approximately 400-600mm away from the trunk depending on size of the tree. If the tree does not move after this we start to dig underneath it to create a ball which can then be lifted out. In some cases, with larger trees we will use a tirfor.
To ensure the best chance of any plant surviving a move, the more root structure and surrounding soil that can be preserved the better chance it has. Larger plants should be staked or tied, sheltered, protected from the wind and kept well-watered.
When digging out shrubs it helps to imagine a plant the same size as yours in a garden centre or nursery and you will need to dig out at least the amount of root and soil that would be contained in that pot.