Ok. An esp8266 averages around 80mA. So your battery needs to be at least 640mAh. You could use 3xAA cells or 4xAA rechargeable cells. They have a capacity of around 2000mAh. Or a 18650 size Li-ion battery which will have a 2000-3000mAh capacity. I recommend a Wemos mini. The standard or Lite version should have enough range, but if not, you can get the Pro version and an external antenna with a higher gain. 2x waterproof ds18b20 temp sensors. Simples!