What size heat mat should I get for my baby green tree frogs? NEED HELP!!!!!!!!

[Lil Krystal]

I have a small terrarium its 25cmx28cm and 18cm in height with two baby green tree frogs. I am buying a heat mat of I can get rid of the heat lamps and move the enclosure in my room
I have 3 options... I've been told to stick the mat on the wall and it shouldn't be bigger than 50% of the surface area. But that puts me in a bit of a pickle.

7W 15x28cm Heat Matt with temperature control

If put it on the side of the tank it will cover the whole surface area so maybe I could only stick half on some how?. But it has a temp control (no idea how well it will work) but I can turn it down when needed or keep it on a real low setting so they don't over heat.
ebay heat mat with heat control


5W 14 x 15 cm Heat mat (add a thermostat??) or2w 7cm by 7cm Mat(with out any heat control)


I have spoken to this seller and he said If I didn't want to spend the money on a thermostat I could go the smaller mat which is only 7cm by 7cm so it doesn't make the enclosure over heat ever. But I don't know if the smaller one will keep it warm enough? but the next size up well be to hot with out a thermostat. I've already spent lots on them so I don't really want to get a thermostat if i don't need it, need it.
Ebay heat mat 5 watt

Ebay heat mat 2 watt


I need a size in between but I can't find one anywhere.

What should I do? Whats my best option guys?

Thanks