A grapefruit tree grown from seed is the worst choice of citrus tree to grow of all the citrus family. However, as Brian wrote, grapefruit certainly can be grown as a container tree. Try to purchase a grafted grapefruit tree, preferentially grafted on a dwarfing rootstock such as Flying Dragon. Even then grapefruit is a large tree so keep enlarging the container as the tree grows. You will probably end up with a 2 or 3 foot wide container. Whatever size container you eventually reach it will not have to be more than 20 inches deep. Good luck. - Millet