I'd recommend keeping them separate. A world map and a regional map serve different purposes. On a world map you want to get a quick overview. Then go to the regional map to see a particular country. One map isn't going to do a good job of both tasks.

So I'd start off with the world map, and think what you want to use it for. If you want a poster map, then start with 11 by 17, or 22 by 17 or some other size in that region which will look good on your wall. Set the resolution to 300dpi for good quality print and then get to work in PS. The regional maps are likely to be smaller (unless you want to have a lot of map posters on your walls) and so you can start with a canvas closer to 11 by 8.5 - letter size. That might end up being a little small, but you can always tile a couple of those together.

When creating the smaller maps, make sure you pull in the world map as your base reference. Place it on a background layer and scale it up. If you add the same amount of scaling for each regional map (always blow the world map up to 400% for example), then your sub-maps will end up being pretty consistent and you'll be able to tile hem together (more or less) to create a patchwork world map at the end of the day.

Does that help?