Now a days chip manufacturers, like Qualcomm, claim that they have built a 7nm chip. Is 7nm really the size of transistor or is it just a marketing strategy? If 7nm is just for marketing and not the size of transistor, then what exactly 7nm is?
Node size is basically a technology standard for equipment makers, in order to align capabilities (i.e. generations) of manufacturing tooling and their requirements which represent several hundred discrete devices in the wafer processing pipeline generally under the SEMI standards organization but the major players (i.e. Intel) have their own standards groups .
What they try to do above else establish is process consistency (accuracy) , in addition to precision . As you can imagine small node size designs require every precise equipment but are also much more sensitive to variations in process so need more accuracy (repeatable real world value) as well. Even if you can make one transistor work you need them all to work to sell a product.
Since a single process step involves many pieces of equipment, they all need to evolve in lock step to support the modern process concepts. Exotic processes may also require exotic materials and much more toxic gasses not well accommodated in current infrastructure, but integral to support the smaller node processes and that is part of the node standard as well.
The process engineers work black magic and they have other meaningful measures of minimum transistor size and density given equipment of a certain category and their own specifications that then get passed to the designers as capabilities of the Fab.
In other words, On the backend a 7nm tool is magnitudes more consistent than the 100nm, to achieve some product design goal, the 7nm marker is the equipment standard needed to hit that node size, something that has a different meaning to everyone in the supply and design chain for process equipment.