Rhododendron roots do grow in the upper part of the soil where there is ample oxygen. They do not range very far, though. It is not possible to make a hard and fast rule about plant height and container size, but a a two or possibly three gallon container would be about right for a two foot tall plant. If you're thinking about using a large container now to accomodate ultimate size, don't be tempted to try it. A large volume of planting mix for a smallish rhododendron will lead only to saturated soil, loss of air space, deterioration of the mixture and quite probably the death of the rhododendron. You need to use a very well drained mixture and gradually increase container size. Repotting every two or three years into a slightly larger container using fresh potting mix is necessary for healthy growth.