The radius of the cylinder around Hildir is approximately 25m. I just tested this yesterday with some pals in the hopes that we could create a large enough workbench zone to protect form mob spawns within the base we built around Hildir's area. It was fairly difficult to get a reasonably accurate idea given the terrain near her in our seed is only 1-4m above sea level, but we were able to set up a square around the cylinder of unbuildable area that measured 50m a side.
I am putting this out there for those who might have the desire to build around the traders without modifying the game to eliminate their 'no-build' zone. Given how much trouble we were having with mob spawns inside the base, it became necessary to actually do some math on things in order to eliminate the possibility that the mobs were spawning as a consequence of the uncovered area within and around the cylindrical 'no-build' zone.
If you're having the same trouble, then you should build 4 fully upgraded workbenches in a '+' shaped pattern at the midpoints of each of the 50m sides of the square or on the corners of the 50m sides. Each of the four fully upgraded workbenches have an active radius of 36m. Collectively, they fully block out the entirety of the area of the 25m radius 'no-build' zone cylinder and ensure that spawns near the edge of this zone are blocked. Alternatively, you can do 4 workbenches with only two upgrades in the '+' pattern and an additional 4 non-upgraded workbenches on each corner of the 50m square.
We love having Hildir close to home for her nice non-portalable fireplaces. However, the base's large area of approximately 25 square kilometers is a breeding ground for food mobs and aggressive mobs. They seem to start coming in and eliminating workbenches from her camp, so the clearest solution at the moment is eliminating the possibility that the game tries to spawn things near her camp and doesn't stop until they find an open pixel untouched by workbenches and other structures.
There are lots of conflicting arguments about what does and does not stop mob spawns. I am unaware of a full-proof solution, save for those which involve disabling the mystical zone and allowing players to build directly within her camp (one fully upgraded workbench directly in the center of the camp would solve this entire problem). Many people recommend fires, flattening/pathing out the ground, and even trying to make an intentional mob spawner within the base that is outfitted with spike defenses to automate material collection. I have seen no clear evidence one way or another on these alternatives to workbenches, but if there is a better way (without altering the game) please give me a shout. I don't want to waste 16 iron for this dwarven lass.