Nylons are shaped to fit the contours of the leg which makes them extremely flattering. But they have little to no stretch in them - which is why they look so long compared to their lycra blend counterparts when you take them out of the packaging.
It's important to take all measurements into account with nylons, as they will not stretch further up your leg if they're too small. Often the sizing for nylons is numbered - and in general this number will be bigger than your shoe size by around two sizes.
As you pull up nylons, the length gives way to volume as your leg fills them out. If you have a very slim leg, then you should consider going down a size, because your leg might not fill out the stocking.
If you have a wider leg, then you should consider going up a size, because the nylon might not stretch high enough up the thigh for a comfortable fit. It's unlikely that you would need to go up two sizes - this would probably give you a wrinkled around the ankle look, which is best avoided!