So the extra small 10x10" size?(says "up to 15 chicks")I use the premier heat plate. Sized for about 8-10 chicks
Heat plate mfg population capacities are as inaccurate as the tiny dollhouse chicken coops. Can depend on ambient temps, but always better to go bigger.
My 12x24 just barely held 16 chicks until they were big enough not to need it.