It changes with system type. NFT and DWC systems are exceptions to the rule since their roots extend and survive beyond the root ball (within the system). Here is the rule. For every 1 foot you expect your plant to be when it is finished growing, you will need about 1 gallon of media. Rockwool needs a bit less area, because the density of the media allows more root growth in the same size container. Most people start out with a smaller container and transplant up one or two sizes during the growth cycle although it’s best to keep your plant in the same container the whole way through if possible. Less transplanting equals less shock and trauma, and less to go wrong. Also, if the plant is in a large container to begin with, it will actually grow faster because the roots will not bind up as quickly.