A bare root planting hole should be no deeper than the roots, but about 2-3 times wider than the root system. Generally a square shape is recommended, but if you've given the roots enough space to be able to spread out and expand, the shape doesn't matter too much.
It's important you do not plant too deep, as this can cause the crown of your plant to become rotten and diseased. It prevents sap from reaching the stems, therefore leading the plant to failure. Planting too deep is one of the biggest causes for bare root failure, so it's important to plant correctly.