Before Marlon Mullen begins a painting, he likes to tidy his work space. He’ll pre-mix his paints — Golden acrylics in recycled pots — and lay out his brushes and canvas on his table.

Kanyeyachukwu Tagbo-Okeke, a 15-year-old boy with autism, painted a massive canvas measuring 12,303.87 square meters — over 132,000 square feet — breaking the Guinness World Record for a ...