The bucket size can be figured if you know how productive your subjected girl(s) is (are). If you have a single cow or goat, it is easy to know after milking it a few times. If it produce under 2 gallons of milk each milking cycle, a 10 liter (2.5 gallons) bucket might do it. If you get more than 2 gallons, you might need a bigger milking bucket. You need some breathing room in the bucket. Its always better to have a bigger bucket so it will never get full. A overflow can create a big problem for your milker. So never let it to happen. I know small bucket are easier to handle, especially for going to shows.