You can absolutely get screwed over with the size of your HVAC. Size matters in terms of heating & cooling. If the HVAC is too small, it will run all afternoon long. The HVAC will work & work until it burns itself out. Then a hefty HVAC repair is needed or the HVAC is just plain done for. Super luxurious or cheaping out with tiny HVAC? Too large of HVAC equipment is no better though. The HVAC is then constantly cycling on & off. The powering up & down is terrible for the HVAC unit. It will also cause disfigure in this process. It also is terrible in terms of efficiency. It is better to keep a steady stream of heating or cooling, then powering totally on & shutting all the way down takes way more power to do. The HVAC will end up costing more when it runs half the time. Most HVAC providers try to size up with HVAC equipment too. What they do is called rule of thumb. Basically the HVAC guy walks around a home & takes loose measurements. He eyeballs it & then pads his numbers up for error. Then, the HVAC professional recommends a size for the home. The bigger the HVAC, the more commission. So those numbers get padded quite high & the homeowner usually gets stuck with way too much HVAC to handle. Do a little research before making the substantial leap on HVAC. Decide what type of HVAC you want & what is the recommended size for your square footage. Take the HVAC company totally out of the equation.