Which small hand sander works for drywall use

Virtually any of the Festool sanders can be used for drywall repair. I would recommend the DTS 400 or RTS 400. They are compact and lightweight and can get into corners unlike a round pad sander. If you would prefer something bigger, consider the ETC EC 125 or ETS EC 150 sanders which have 5" and 6" diameter round pads respectively. The "EC" sanders have a brushless motor resulting in a more compact profile and very light weight.