dude if your vegging for two weeks you could get away with a one gallon .. not recommended but you could do it. then again .. they say a gallon per foot. but I've been vegging for 3 weeks in a 6x6 inch square pot.. like 3 liters almost a gallon. before flowering i plan on moving only up to a 9x9 inch pot.. small i know, but i try and keep it compact.