Just size it to your boil size. You could go a bit smaller sure, but having it full size allows you some flexibility in those unknown cases when for example you need to store your full boil volume because of something happening to your kettle or what not. Also maybe one day you'll want to boil both at the same time, or do a partigyle, or collect a bunch of hot water from your chilling process. I wouldn't make it smaller than my kettle for reasons like that.