Whatever size fits about 1/3 of the bottom of your cage. UTH's all work the same way, since they'll be hooked up to a thermostat to regulate the heating.
Check this out for the equipment necessary and setting up your tank (though I personally HATE foil tape, because it sticks to everything and doesn't come off... using regular paper tape is just fine, using a thermostat) A ball python would also need more hides and more cover than that https://ball-pythons.net/forums/show...-With-Pictures!