A sheet of glass or acrylic that you'd use for picture framing works equally well as the trendy Gelli Art plate. The benefit with the former is it's much cheaper and it's likely you have an existing piece you could repurpose. The Gelli Art is more portable and easier to clean but the printing quality from both are equal.
A 16"x20" piece of acrylic provides plenty of working space. Here 3 colors of acrylic have been rolled and then imprinted with texture (plastic rubbing plates).
After the first press spritz the plate with water and press with a fresh sheet of watercolor paper.