For compressor sizing it is necessary to figure out how much air you're going to need. That is... the air requirement for all the air tools, all the blow guns, all the air cylinders, all the air operated diaphragm pumps, all the everything that is in your plant or workshop that uses (or has the potential to use) compressed air and whether or not that compressed air demand is going to be constant, or change as your tool use changes, production using air equipment increases, and so on.
And, though there are many manufacturers of compressors world-wide, all are focused on selling their own products, which may impart a certain bias to the information that's available from them about their compressor-sizing, you think?
The above link you may have just visited took you to a short list of some in-plant compressed air applications, this to help you determine which of those you have or are planning to have, equipment that will be consuming your compressed air and needs to be accounted for when compressor sizing.
If your compressor is for continuous air use then start making a list of everything in your plant that uses air, and leave a space beside each to note the expected CFM demand for each. Better yet, use a spreadsheet so that the air consumption numbers can be set up to give you a running total of the air you'll need.
Know that compressed air consumption of tools varies depending on the size of the air tool, it's speed, and the efficiency of that air tool. You need to get the specific compressed air demand for that air tool from the manual or the manufacturer of the tool for accurate assessment of the overall compressed air demands of your shop.
You may find the chart below handy in approximating the CFM requirements of an assortment of standard air tools. This is a guide only. Your actual consumption of compressed air will vary, depending on your tool size and design.
Consider "factoring in" the highest air consumption figures you find for the tools you expect to use. You may oversize your air compressor capacity by doing so, but under sizing the compressor you purchase may bring greater long term costs to your plant as you grapple with air tools or air cylinders that do not run as expected due to a lack of compressed air supply.
Some folks don't consider an air-driven blow gun as an air tool, yet these can sometimes use more compressed air than expected.
Air driven automated production machinery can consume far more air than some air tools as air operated production equipment with valves, air cylinders and other air actuators is often run for 2 or 3 shifts per day, with uptime measured in weeks. Each piece of equipment may have dozens of air actuators or air cylinders operating at high speed.