When it comes to air conditioners, buy the biggest one you can afford. Bigger is always better because that means it has excess capacity, so it will last longer.
This is false! Air-conditioners work primarily by reducing the humidity. An over-sized air-conditioner does not run for very long at a time, and allows the humidity to build up, making you feel less comfortable. Aim for right-sizing not over-sizing your HVAC unit, and it will actually work better!