The size of the mat will largely depend on the space available and how much of the floor you would like to cover.
If you like your flooring or have a smaller kitchen, accent mats in high-use areas can improve comfort and protect the floor. Alternatively, a larger, more open kitchen might benefit from a big, warming, and eye-catching rug placed right in the center.
For narrow kitchens or spaces between furniture such as the counter and island, a runner rug works best. Just make sure to leave enough space either side, so it doesn't get in the way of cupboards and appliances.