When I was in school for geomatic's we dealt with matters of scale and detail in a number of ways... one of our exercises was to render a square chunk of a map at 3 different scales... one was a plot of land with a number of roads, rivers, and railways in it... and the other was a detailed map of forestry with different areas of forest... the stroke sizes we were allowed to use were defined for us, which severly limited the possibilities of redrawing the area's using the same detail.

The challenge of the exercise was to make them legible at all three scales, and display as much information as accurately as possible. In the forrested area you were forced to simplify the forest clumps, often smoothing them out and leaving out the smaller bits... It's something that isn't often realized nowadays with vector graphics, since they scale perfectly it is common to take the highest resolution and just zoom out... but using a highly detailed (not smooth) stroke and zooming out, it often comes across as bold, blurry, and out of place... in that case it's usually time to see about changing the scale, or creating a simplified copy of your data...

The roads/rivers/railways was even more interesting, since as the image got smaller, it was quickly apparent that the lines that were distinctly separate in the large version became crowded and even overlapped in the smaller versions... The only way to keep these closely running lines legible is to move their location, to space them out so you can still see that there is a road, rail, and river running side-by-side, and that they do not cross... This is what RobA means by the information being a priority... In a map displaying something like transportation routes, identifying the interactions between the roads rails and rivers is imperative, and in that case, to keep the small map useful, it requires you to modify the geographic locations... This specific example is just one of many reasons why cartographers are still required... we may have the technology to display our data in a map, in a scalable lossless format, but conveying what we want to convey is still the top priority, and in map's it requires some experience to decide what to show and what to change.

For your request I would suggest working in a broad, generalized concept before filling in the details... Place your current region in your realm, zoom out, and get an idea of the generalized "outline" of your finished work... then work out from there... I believe it's much easier to work top down than bottom up, but the top doesn't need to be much more than a general idea of the layout.... figure out where the major geographical features will be, things like mt ranges, major rivers, swamps plains, and forests... then decide where the largest of cities will be, come up with some major trade routes or roadways... it'd be good to define political borders as well, since they can easily be fairly generic...

From there you can go into more detail, maybe starting with the areas immediately around the finished region... coming up with some of the finer aspect... World Building can be pretty much as easy or tedious as you would like, so it's really up to you as far as what you want to get out of it...