a 20L would last you awhile, but you'll end up moving him/her to a 30 gal breeder tank before adult hood.

the 41 qt tubs on my rack are larger then a 20L tank, it is just not as high as a tank, which a ball python could careless about.

your best bet is to use tubs until he/she reaches her adult size. while they are in tubs save up some money and buy a nice plastic display cage from RBI or boaphile. those are 100x easier to maintain heat and humidity.