What exactly do you mean by fig tree? Do you mean a ficus tree, or a fruiting fig. Fruit trees don't really grow indoors unless you have the super high light of a conservatory. Well, they may grow for awhile, but they certainly won't fruit. Your typical ficus tree ( ficus benjamina), weeping fig, is another matter entirely
You can buy them any size you need to fit the spot you want to put them - just give them good light, near a bright window. Don't be in a big hurry to repot - the soil the plant comes in is good - but when you do repot, use a fast draining potting soil - I like cactus mix - or a soilless mix for fine-rooted plants. Fertilize with a balanced fertilizer, 6-6-6 or 10-10-10 for instance, once a month during the growing season, laying off for the winter.
