Carrageenan is also known as "Irish Moss", even though it is made from a special type of seaweed. Actually, we think the seaweed is called Irish Moss but enough trivia. It is used in this business for making the jelly-like solution called "size" that marbling paints float on. Marblers use either this, or Methocel, which is a sawdust by-product. The main advantage of Carrageenan is that most folks find it easier to float their paint on it than on Methocel, making it easy for artists and better for beginners. The main advantages of Methocel are that it is less expensive and it keeps longer, especially when you have a sizing pan so big it cannot be refrigerated when not in use, and it is hot and humid out. Try both, see which works better for you in your situation with your skill level.
Mix 2 tablespoons to a gallon of water. Best to do it in batches in a blender for the most even consistency.
For a square yard 36" x 36" you will need to fill a 38" x 38" tray with about 7 gallons of size.