Just posting this because it looked like a cool idea.
I would think you could adjust the amount to your needs.
Buckets o' Blood 2
This is great for the 'hands-on' type of blood, and (from what I hear) also works well in bath-tub sized proportions.
1 package plain gelatin or 1 package red colored JellO
2 bottle red food coloring (especially if using plain gelatin)
1 tbsp green food coloring
1 - 5 gallons of water (depending on desired consistancy)
Directions are simple: Follow the instructions on the side of the jello package, but double or quadruple the amount of water needed, and don't add any sugar. Doubling the water gives you a very slimy, gloopy jello which doesn't look a lot like blood, but can be fun to get kids to stick their hands into at hallowe'en parties. If you use 5 gallons of water, you're going to have quite a thin runny blood, great for pouring over bloodied bodies in bathtubs or splashing on walls (especially if you can hose down the walls after - I wouldn't recommend this in your livingroom or parent's bedroom). You can play around with the recipe to get the desired consistancy - gelatin is reasonably cheap and available in almost any grocery store.