Check with your local nurseryman for the species you intend to plant. Planting holes should be wide and deep enough to accommodate bare rooted trees without cramping the roots. When planting root-balled stock, the hole should be at least a foot wider than the diameter of the ball. If the ball is wrapped in burlap, untie it and roll it back from the tree after planting. Plastic burlap and plastic bag wraps don't decompose in the soil, so completely remove these.
Many people make the mistake of planting their tree too deep.
The root collar should be at ground level. It is better to plant the treed little high than too low. Use topsoil to fill in the hole. Water the tree immediately after planting and continue to water it periodically for two seasons. Trees need a soaking about once every seven to ten days.