Is there a general rule for how to plant / how big to dig the hole for the plant?

Generally you should dig a hole in the soil twice the size of the pot or root ball.
Next, dig over the subsoil left in the hole and mix in any available compost.
Add a small handful of slow release fertilizer, such as Trevena Cross General Fertiliser and then turn over the soil again. (If you place the plant straight on top of the fertilizer its roots will soon burn out if the plant becomes dry.)
Place the plant in the hole and back-fill with the remaining soil.
Water well and continue to water well for the duration of the season of planting.