Generally speaking - Carb sizing is about determining the size that is big enough to adequately feed the engine but not so big it will reduce the response and metering ability. Most sizing formulas use engine size, rpm and volumetric efficency to determine the theoretical volume of air an engine can pump if it were an air pump. Very simple calculation. The Cubic Feet Per Minute (CFM) number you get from that calculation (or chart based off that calculation) will get the job done. A smaller rated CFM carb will still run, but it will hold you back from a performance standpoint. A somewhat larger carb will allow you a little more HP but it will be less responsive. If you go much larger in size above what is on the chart you will loose response and metering ability.
Carbs are rated at the CFM they will flow at a given pressure drop. 2-barrell carbs are rated a different pressure drop as compared to 4-barrell carbs.
Vacuum secondary type carbs are more flexible so they work better than a full mechanical secondary carb when they are both equally "oversized" based on the calculations. Vacuum secondary carbs are generally preferred for automatic transmission cars but will also work well with manual tranny especially for a street car. Quadrajets are very flexible due to the mechanical secondary with a vacuum operated air door on top. The advantage here is very good drivability over a wide range of conditions regardless of the size rating. Quadrajets are more sophisticated than some other carbs but tuning parts are harder to find for a quadrajet in my opinion. Holley parts have allways been easier to find and more people seem to know how to modify Holleys as compared to a Quadrajet.
Manual secondary carbs are good for racing and some higher HP stick cars but they are less forgiving when it comes to oversizing. When you stomp the gas, the carb will open all the way and the accellerator pumps will empty regardless of the true needs of the application at that particular operating point. Fun, but not allways the best choice.