You could maybe get away with 40L batches but you'll need to pay close attention for boil overs. The half barrel kegs are the best to use they hold about 58L insted of 50L I find the 50L ones make great HLT's but I prefer the bigger ones for boling. To answer your question though I'm not sure how much they'll comfortably hold for the boil but I would assume 40L isn't impossible.