The size of a hand sander can vary slightly, depending on the individual model and manufacturer.
However, the most common size is 80mm (3″ approx.) in width by 230mm (9″ approx.) in length.
It is useful to know the size of the hand sander you are using so you can cut the sandpaper to the correct dimensions. Alternatively, if you are using pre-cut sanding sheets, then you will know what size to buy.