i use simple flxwatt I found the 11 inch work good for bps and on my smaller snakes(none bp snakes like corns and kenya sand boa) I am going with a 4 inch wide.

You need ot consider the need of the snake you will be placign the snake and the abient air temps . so if the room stays warm all the time then a smaller hot spot but if it cooler ( like due ot ac then a larger warm spot is needed to help keep the entire cage warm. but wither way need soem type of thermostat control so your not cooking the snake due ot a power spike, heating pad issue or if the sun shines on te tub and raises the temp 10 -30 + more temps than normal.