How to do an open area

[noob]

It depends on how fast you are at improvisation on the fly.
Do one large scale map and very important elements.
Then have a random terrain generator(with one seed) for each kind of terrain you use and when the players says "we start researching at place X" and other stuff of this kind you use the terrain generator and you generate the local area(Or if you are not in procedural generation with a computer you just invent on the fly half featureless bumps with two trees or you just describe a place and play without a map of the surroundings)
Then very important elements(like the local town and stuff of this kind) map them yourselves and do not try having them too much huge.(this will make you loose a lot of time and not necessarily make the game better)(if you see it takes you more than ten minutes to do something maybe you are putting too much work in it and should rather see less big)
Now tell the players they can get everywhere they wants(as long as they can get to those places).