Fallout 76's map is four times bigger than that in Fallout 4. The entire map is divided into six sectors. They are divided by their topography and enemy levels. Your character will be able to travel through mountains, swamps, wastelands, ruined towns and floodplains.
The map in Fallout 76
The six in-game regions are divided by their climate and enemy levels. A mountain range will go through the middle of the map. That area will also feature abandoned farms, mills, factories, mines and towns. Those terrains will be a good source of resources and items.
The area in the center and in the west of the map is called The Forest. Right below it you can find Ash Heap. Above The Forest you have Toxic Valley. The middle of the map is called Savage Divide - it spans from the north and goes to the south (wastelands with less buildings and activities). In the east you can find Cranberry Bog and The Mire.
The Forest is located in the east and in the middle of the map. There, you can find a lot of buildings and other places that can be explored. Vault 76 is located in the middle of that region. The Vault is surrounded with greenfield. Mountains can be found in the east. The Vault is connected with the nearby river by a road, probably to make collecting water easier. The river on the left is probably the end of the map.
Since the Vault 76 is located in The Forest, it is highly probable that this is the starting area of Fallout 76.
The Ash Heap
In the Ash Heap you will find a lot of bigger and smaller locations. These lands were probably the most damaged by the bombs. This also means that the radiation in that place is high. Entering these lands can result in a radiation sickness. It is also probable that this area will have better weapons and resources.
Cranberry Bog has a lot of empty facilities. This region will probably be the most challenging both weather and level wise.
Morgantown, along with the airport and Vault-Tec university, can be found in Toxic Valley region. This area may be filled with monsters but they shouldn't be at a high level. Fans speculate that this could be the second area explored by players.
The majority of the map consists of deserts. There, you can probably find sandstorms and other weather phenomena. At the bottom of the map you can find mountains - avoid going there during a storm.
Rain and storms will probably appear frequently on the swamps. Also, there are a lot of facilities (e.g. Harpers Ferry, Bramwell and Martinsburg) that will be probably visited by a lot of players