Yeah, I know a lot of gimp users on here, and while they sometimes complain about slowdowns when running certain scripts (like fractalizing paths when they have dozens of rivers) I don't think i've heard anyone complain about slowdowns due to filesize.

I'd say you should be pretty good to go - in fact, it might be worth it to try pushing the limits on a file just to see how far you can take it before you need to concern yourself, that way you can put less effort into worrying about files and more effort into the work itself

I did that not too long ago, and it's a very subtle psychological difference, but for me it was noticeable
(using photoshop here, but the underlying principles I'm talking about are the same for any software out there. know what happens when you get close to limits, and you tend to be more efficient and practical)

That all aside, what Ascension said about roads is pretty much all I've ever learned.

The only thing I can add is I like to think about it from a visualization perspective (will they actually look good on the map, and is it necessary information for the purposes of this map?), and keep in mind the economy of the realm they are in - can they afford roads? is it a small enough region that you want to show iffy dirt roads and such, or just stick to well maintained ones? Picture yourself as a merchant trying to use the roads, would you actually take the long way around the forest like the road does, or do you know that it's safe enough to take a shortcut? Why would people use the road if it doesn't take them somewhere worth going? all that stuff.