It doesn't matter on size, all will produce the same amount of heat, accept larger mats will cover more space. You can take the foil off, there's no need for it really. Heat mats are not designed to heat the air and create more of a hot spot animals will seek out for heat. However, when it heats damp substrate some heat will go into the air. Use a digital thermometer, place the probe directly on the mat to get an accurate temperature reading, as well as a thermostat probe. My frog moved the thermostat probe the other day and the mat reached 100F. The thermometers you stick on the side of the cage will not accurately measure the temperature of the mat as they measure the ambient temperatures but you need to measure the temperature of glass underneath the substrate, where the frog will be. Although the ambient temperatures are low, the frog will burrow down to the mat to heat up. Don't use bulbs with heat mats unless the mat is on the side, as then there will be heat coming from both directions so it may confuse the frog