I have the first 2 Anova models and have used either with nearly 5 gallons of water in a Cambro container for 48 hour cooks with no issues. I have not had a need to go any larger.
To do the calculations you need to know the heat loss and dimensions of the cooler. I honestly do not think you will have an issue. Do a search for coolers used fro Sous Vide to get an idea of those others have used. Here is what I use fro bigger cooks.