the best choice is change 3 times pot during the growth cycle. thats help to devolve strong root system, this happen becouse every time you make a trasplant, roots reach border of pot and if you pass from jiffy to last one, all the soil wasn't occuped by roots but only border and a little center part. so, if you think the last one is 20L, you can start from jiffy >> 8lt >> 20 lt and creat onion like situation for roots step by step.