It all depends on what you will be growing.
3-5-gallon grow bags works well for a single plant like pepper, cucumber, cabbage, peas, or a combination of smaller leafy greens.
5-15 gallon grow bags well for Tomatoes, potatoes, flowers, onions, garlic, carrots, beets, large herbs, small fruit trees.
There are 100, 200 gallons grow bags available in the market that can be used as an alternate to raise beds.
For large trees, there 300 and 400 gallons bags available in the market.