The ideal size for a golf hitting mat depends on the space you have available and your specific needs. A small hitting mat (around 3x5 feet) is sufficient for practicing your swing and working on your fundamentals. If you have more space, a larger mat (5x10 feet or even bigger) allows you to move around and practice different stances and shots. Keep in mind that it's essential to have enough room for your swing and any practice nets you may need to use.