How do you decide whether to go with a company that claims to be the biggest in town or one where the owner personally worked up your estimate and will be on site as part of the install team putting in your new system? It bears mentioning that there are both good and bad seeds in large and small size companies so the onus is on you to do your due diligence. Where to look? One place you’re likely to get 
some mixed signals is sadly the manufacturer’s websites. Why? Many reasons, for example when you put your zip code in the mfg.’s dealer locater the companies that pop up first will not be the ones closest to you nor are they the best contractors but rather the ones who have paid a fee to be a preferred dealer. That's not all bad and there is some value there, but use it only as one piece of information in selecting your contractor. Here’s another reason why. The name of the game is sales, manufacturers want to move boxes, they want to sell their equipment, they want it out of their warehouse and on the side of your house or up in your attic or down in your basement depending upon which part of the world you call home. Manufacturers look favorably upon contractors that move more of their boxes than those that don’t. The point here is bigger isn’t always better just like system sizing. The bigger companies are not necessarily better or worse. However many of the bigger companies faceless entities, divisions of large national corporations with no owner on site. Why does this matter to you? Suppose you have an ongoing problem after the installation that you cannot seem to get resolved. It can be extremely helpful to visit face to face with the person whose name and reputation is on the line and on the business. Although we personally prefer to use smaller “mom and pop” shops ourselves when we hire other trades you should still expect the smaller contractor to be professional, drive a neat lettered truck and be wearing a company uniform. If they show up in a mustang convertible wearing a tank top, shorts and sandals send 'em to the beach.
How do you know you have the correct size furnace and air conditioner for your home? The answer is heat loss / heat gain calculations. When done correctly they are extremely valuable in determining the correct amount of Btu’s required to heat and cool your home. The key word is correctly. Conversely when done incorrectly all kinds of bad things can happen. The old garbage in garbage out cliché comes to mind here. There is so much information that needs to be collected and input that the room for error is huge. On the plus side these calculations are now done on computers increasing accuracy and saving time. Unfortunately there is no shortcut for collecting all the information from the house you want to run the load calculation on. Doors, windows, walls, ceilings and insulation must all be measured and condition noted. Ductwork must be measured and inspected for proper sizing, noting its condition and insulation r-value. The type of building and construction materials must all be known such as: amount and type of insulation in the walls and attic, type of windows and which direction they face, their frame material and number of panes, etc. There is more but you get the picture. The challenge is in older homes you don’t have access to a lot of this information, for example what type and r-value insulation is in the walls if any? So what happens is guesses are made; often uneducated guess’s made erroneously. An experienced estimator who has been in the industry for years and knows construction can usually get it right and do just fine. Problem is most estimators do not fit that profile. The point is I can take the same house and enter it into my computer load calculation software using different insulation values and construction materials to name just two variables and come out with two completely different size furnace and air conditioning systems.
Bottom line, never were the words Garbage in garbage out more fitting. . Your contractor should perform a heat gain / loss calculation and use it only as one of many pieces of information to properly size your equipment.
Every company claims to have the best guys to install your new system. But how do you really know? This is a little tricky as you won’t see the install crew until they show up to install your system and even then you won’t know what kind of work they do. Your best defense here is a good offense and that means references, references and more references. When you ask for references get a variety, preferably from jobs completed in the last two to 18 month. This will also give you an idea on how your prospective company treats its customers after the sale. The most important day in the life of your system is the day and the way it’s installed. I’ve seen the best equipment fail prematurely due to poor installation practices and also observed lower end equipment exceed its design life due to a properly done installation.
Today most major Air Conditioning manufacturers offer a 10-year parts warranty if the equipment is registered online with them within X number of days of installation usually 45-60. You want to be sure this is done. Secondly you should be able to purchase (from your contractor) a 10-year labor contract offered by the manufacturer during the same time period. It's generally a good idea if the price is right, usually 300-500 dollars depending on the equipment. Most Air Conditioning contractors offer a standard one year labor warranty with their installations, this can sometimes be negoiated up to two as part of the negoiation process with the salesman.
Summary:
Get multiple bids from small and larger contractors
Go with a brand name manufacturer.
Stay away from contractors that sell private label equipment.
Use the manufacturers website to have some understanding of their products. But not necessarily to pick your contractor.
Ask friends and family for referrals.
Ask to see the results of the heat load calculation and receive an explanation as to how the contractor arrived at the size of the system they want to install.
Thoroughly check the references you receive from your contractor.
Get a ten year parts and labor warranty, someday you'll be glad you did.
Lastly it should be common sense but it needs to be mentioned, use only licensed and insured contractors just ask and they should gladly show you their credentials.