To grow a tomato plant in a grow bag, you'll want a bag that's at least 5 gallons in size, but preferably larger, around 10-20 gallons, to accommodate the plant's root system and provide ample space for growth. However, when it comes to grow bags for trees, especially larger fruit trees like citrus or apple trees, you'll need significantly larger bags, often in the range of 20-100 gallons or more, depending on the size of the tree and its root system. These larger grow bags provide the necessary room for the tree's roots to spread out and establish a strong foundation, ensuring healthy growth and fruit production. So, while a 5-gallon grow bag might be suitable for a tomato plant, you'll need to scale up significantly for trees to provide them with the space they need to thrive.