Well, there are a few things to consider:

First, what is the extent of the map, and how big are the actual features being represented. If you don't know the extent right from the beginning, you are going to be in trouble later for all sorts of reasons. Before you draw anything, you should know where it is on the globe, and how much area it covers. Once you know that, you can calibrate everything else based on it. You can draw any features big enough to be visible on the map at their real size then without any particular difficulty.

Second, if the map is meant to look like a real person in the setting made it with real tools and materials, then you need to have an idea of what those tools and materials can do. This will give you a minimum resolution of sorts that you can produce (Which will probably be quite a bit lower than the actual resolution you work at if using raster graphics). If the notional cartographer has only a particular set of pens, than that defines the range of line widths you can use for instance and so the size you can draw a symbol and have it be recognizable.

Third, consider how the map will be used, again, this may be notional as a nautical chart of Middle Earth will never really be used as a nautical chart, but you still want it to look like it could be. A wall hanging map will probably be seen from further away and so need larger symbols for instance.