So here's my proposition: Break the gameworld into small, medium, and large sections based on squares. Then you could have a bunch of areas created for random encounters (some desert areas, some mountain areas, some abandoned and ruined urban areas, a few farms, etc) and some small unique encounter locations.
Fallout 3 will be used as a base size, imagine Fallout 3 as a 10 x 10 map, so 100 total squares.
I'll diagram Fallout 2 based on this.
Please view the image: http://i9.photobucket.com/albums/a96/The_mango55/Games/FalloutMap.jpg
Besides the maps shown there would be 15 one square maps for the http://fallout.wikia.com/wiki/Special_encounters. There would also be 15 random encounter squares (3 desert, 3 urban, 3 mountain, 3 farm, and 3 others). If you aren't familiar with random encounters in Fallout 1 and 2, you can encounter various things such as groups of enemies/animals, caravans, prospectors, raiders fighting farmers, slavers capturing tribals, gangs fighting other gangs, and so on.
As you can see, my idea would make the Fallout 2 map 53% larger than Fallout 3 (including random/special encounter squares), despite covering a MUCH larger area, and while these areas are centered around cities or major points of interest, they still give room for exploration.