After reading the Mike Venezia biography about Jackson Pollock I showed them a 2 minute video clip of him working on one of his big splatter paintings. I was sure to point out Pollock's messy work boots so the kids knew what kind of fun awaited us.
Then I found the biggest pieces of paper I could and we went outside to "work". (Really we went outside to get messy and laugh.) The paper was about 3 feet x 4 feet.
It was great fun for all three of us to make our own large-sized art.
The one thing I didn't plan for was how to get these projects into the house for drying and safe-keeping! If we had done this project in the morning, the paint would have had 10 hours to dry before we picked it up. We did it around 4:00, though, and had a tough time maneuvering through doors and down stairs without smearing the paint.