I would, personally, feed that snake 2 adult mice per meal if he would take them, but one adult mouse would suffice most likely. Weight would be a better measuring stick than that picture for me.

A rheostat is, essentially, a light dimmer. When you set it, what you're doing is controlling the amount of power what ever you're powering is receiving. They are ok, but are not the most reliable and also don't adjust automatically with any changes in ambient temp

Thermostats you can set to a certain temperature and when your heating device warms up the thermostat probe to that temperature, the amount of power received changes(or cuts off completely in the case of on/offs). When the probe gets colder, the stat gives more power to the device.