Obviously you can grow a bigger plant in a 7 gallon than a 5 gallon. It isl not an issue in most indoor rooms because of height issues so 3 or 5 is plenty big enough. I like using 5s but not filling them all the way, the width is the factor. So if you try 7s because your grow room ceiling is 10' high (lucky you) make sure you get the squat, short, and fat buckets and not the tall skinny buckets. Weed roots like to go horizontal (the feeders) more than vertical.