Nameless Sprite Editor (NSE for short) is a good tool for adjusting and making overworlds.
Type: Sprite and Icon Editing
Compatible with: Pokemon Ruby, Sapphire, Emerald, FireRed, LeafGreen
Supported OS: Windows
The tool Nameless Tile Map Editor (NTME for short) is created for editing – adding the tilemap for Pokemon Gen III games.
If you think your overworld is too small or too big and want to resize it to your conditions, use Overworld Changer.
Even one small detail like the foot print of Pokemon can also be edited with Foot Print Editor (FPE for short).