I use maps all the time, and for an area of say 2x2k I tend to make a base TIFF of 4x4k (grey). I use scales that are set from the start, depending on what the area specifics are. What Oshyan says; it depends on the level of accuracy you need. My smallest paths are sometimes only 1-2px wide, but sometimes fall away. I always test the width I draw roads/paths waterways first and measure them in TG. Say I paint a line with a 12px soft brush, and the line in TG is 5m wide, and I need a path 2m wide, I can do 2 things, reduce brush to 6px or so, or adjust in TG by color adjust. I use a mix of both, just depends on what I need.
But greyscale 8 bits is fine (and not heavy at all), and you can combine 3 grey maps in one RGB file, split again in TG.