Of course since I use vector instead of an image editor, I don't think in pixels per inch, since pixels (in vector) mean nothing until export. However, I think my perspective still might be helpful even to image editor users. I think about how many inches (vertically and horizontally) do I want the map, and is the map intended to be printed or used in a virtual tabletop application. If a map is intended for print, most large format printers are limited to 36 inches wide, though a 42 inch wide inkjet printer is common. Larger printers exist, but then it gets really expensive to print at larger than 42 inches. So once my inch scale exceeds normal printers, than it is only practical to use a virtual tabletop application for map display. This thinking largely applies to encounter scale map (1 inch = 5 foot scale).

When designing a map that is not intended for encounter scale, such as a city, regional, continental or world map where 1 inch might equal 10, 20, 100, 1000 feet, or miles per inches, then I limit to the largest normal and practical print size which might be 24 x 36 or 36 x 36 inches.

So if using Photoshop or other image editor, using my line of thinking, once you determine how a map will be displayed (print or VT-ready) and what is the maximum practical display size in total inches, then and only then what resolution you prefer to use becomes the issue. If you intend to print, 300 ppi is the goal, then of course, you need to calculate 300 times the maximum inches you need to display your map. If creating a map intended for virtual tabletop, which one becomes an issue as Roll20 (for example) allows you to display up to 72 x 72 inches at 72 ppi, whereas many other VT apps are limited to 24 x 24, or even VT apps for phones might be limited to 2048 x 2048 pixels total.

Just for clarification, the primary difference between image editors and vector is that once I complete my map design in a vector application, I can export the file to whatever resolution I want. The same map design can be exported at 72 ppi or 100 pppi (intended for VT-use) or 300 dpi intended for print. Using image editor software, you are forced to determine max resolution per inch at the beginning of your design, but knowing how you intend to display your map still matters in helping you determine scale and resolution.