An industry is any marked section of track or tracks in a town. It can be as large as several tracks, or as small as a single location on a track shared by multiple industries. Industries should be marked so the train crews know where cars destined for each industry should be placed.
On my own layout, I have cases where an industry is a single track. I also have one industry, a cannery, with two parallel tracks and 10 separate places for cars where the first two spots on the far track are considered the "cannery oil" industry, and heating oil tank cars must be placed at that far end of the siding.
The length of an industry spur, as far as SwitchList is concerned, is the amount of space available. If you declare an industry to be two tracks, feel free to add the lengths of the two tracks to give SwitchList an idea of the industry's capacity.