Not unique to Guatemala, in the tropics the climate is much more a function of altitude than season. There are wet and dry seasons but the temperature does not vary much throughout the year. How much? Five degrees Celsius is normal. Thus, you can pretty much pick the temperature range you like and then look for a place to live at an altitude that offers this range.
Much of Guatemala is between 1500 and 2000 meters of altitude including Guatemala City. At 1500 meters, the average high temperature is below 30C and the average low temperature is above 10C, 24/7/365. If you head toward one of the coasts it will be warmer or go to a higher altitude such as Quetzaltenango (Xena) for something cooler.