For me that appears to be pretty small, most likely the average tablet area across all users is going to be considerably bigger.
However, as Lyawi pointed out before, use an area that makes you feel comfortable - though again, with an area that small you might experience a shaky cursor way more than people with bigger areas -
However, as Lyawi pointed out before, use an area that makes you feel comfortable - though again, with an area that small you might experience a shaky cursor way more than people with bigger areas -