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any monster gives me its size and from this I determine its size on the grid! at that point I have the center of its position and with this I can calculate the distance from PG! In the case of the swarm, however, this does not happen and therefore I cannot understand where and how these creatures are on the map and how to calculate the distance from the characters!
Let's take an example:
vampire bats; swarm size; two of their attacks hit "Characters within 10 meters". At the moment this enemy comes out I can also think of a point on the map where they appear but then not having a defined size I wonder how to manage it! Or if that distance does not indicate the radius (or diameter) of the swarm itself!?

Moderator Action: Moved the thread to the GM only section because the answer may contain spoilers.

Sorry that my it took a little while before I answered, but I wanted to run it with Tomas first just in case I haven't misunderstood it myself (we are all humans after).

A swarm can vary in size between Normal (2x2m) to Huge (8x8m). A swarm doesn't block movement so technically the size is only there so the players know where they need to be in order to attack them. They can even be inside the swarm

When it comes to how large the swarm is in a given location, just let it be as large as it can be. In a room 8x8m or larger, let it be 8x8m (huge), if it is in a 4x20m corridor let it be 4x16m. If it is in a smaller area, then just use whatever size you can. When a swarm moves it can pass through tiny passages.


For vampire bats and their 10m attacks:

Swirling Horror! The bats swarm around their victims at a frantic pace. Everyone within 10 meters suffers a fear attack.

and

Mass Attack! The bats split up to attack all characters within 10 meters. Each victim suffers D8 slashing damage and the bat swarm heals the same amount of HP from drinking their victims’ blood.

This just means that the swarm split up and attacks everybody within 10m from where they started from. Bats have a movement of 24m so after this split up, you can move the bats pretty much wherever you want after the attack, back to a big 8x8m square (or smaller if the location isn't huge). The split up is just for the attack, they don't stay split up.