A marbling size is a suspension mixture which your marbling paint floats upon. I used methocel to create my marbling size, which will thicken the water just enough so the paint will rest on the water’s surface. There are other products on the market that you can use to thicken the water, but I prefer methocel because it’s quick to prepare and the mixture will keep for 3 days at room temperature or a week when stored in the fridge.