I'm guessing you mean how to work out what's a reasonable distance for spacing of landmarks and the like?

The main thing to keep in mind is that a pre-modern society is built around (effectively) walking pace. While you can travel in short bursts more quickly on horseback or the like, most people won't, and horses tire quickly, so the standard distance measures will be based on a speed of 3-4 mph. A league is probably about the distance you can expect to cover in an hour, unless you're travelling very light or on a very good road, etc. You're probably only going to travel in daylight, and you're going to need to stop for food, drink, rest, etc. so if you do pretty much nothing else but walk in a straight line taking that into account the average person can likely expect to cover 25-30 miles in a day. It might take a D&D party longer to cover that distance if they're stopping to fight random encounters, prepare spells, heal, etc.

People are of course savvy business operators and will have worked this out. If two towns are a few days' travel apart there will almost certainly be inns and resthouses along the way catering for weary travellers, unless you're in the middle of nowhere. Coaching inns will probably be quite regularly spaced.

A similar principle can apply to villages. A village which aims to be more than self-sufficient will need to get its produce to market, which means it needs to be within striking distance of a town, preferably a town you can get to and back from in a day while still getting a full day's trading in while there - so almost certainly no more than ten miles at the outside. Any settlement more than ten miles from another one is likely to be pretty insular, isolated and possibly backward. A town probably couldn't survive more than ten miles from the nearest village unless it had another means of supply (a port, or magical means of food production).

Equally though you don't want towns too close together unless the land is really fertile and high-producing. There is a website - Medieval Demographics Made Easy - which goes into a bit of detail about reasonable levels of population density and the number of towns a given bit of land can support, etc. which might be worth checking out if you haven't already seen it. Those towns will tend to be fairly evenly-spaced: not completely regular, of course, and they'll tend to occupy strategic locations rather than just being plonked in the middle of nowhere, but within a smallish margin of error.

Of course, if it's D&D you can ignore the rules, and you certainly don't want to be mapping every last village anyway, only the interesting ones, but it's useful to have an idea of what the rules are before you start breaking them or skipping over elements of them.

As far as getting the scale on the map right, I'd probably try to work out where you want your features to be in relation to each other, and then work out the scale in accordance with the above sort of guidelines. Obviously there's a degree of common sense involved: if you decide that two towns are fifteen miles apart, there probably won't be an entire ecosystem in between them with mountain ranges, lakes and the like. But you might decide you want to put that in there, and then adjust the scale accordingly, on the basis that you're ignoring all the boring uneventful towns and villages in between.