Small random fact: The largest TR in DBD.
The largest possible Terror Radius in Dead By Daylight is achieved with the following items:
Doctor with Calm-Carter's Notes+Calm II: Increases TR in Treatment Mode by 12 metres
Distressing: Increases TR by 26% at T3
Monitor and Abuse: -8 metres on TR when not in a chase, +8 when in one.
With this all stacked up: When a Doctor with the 2 better Calm Addons is in Treatment mode and in a chase, the total TR is 32+12+8=52. Combined with the TR increase of Distressing, which adds 26% of the TR, you get 52+13.52=65.52 metres. To put that into perspective, Tier 2 Empathy reads the auras of injured Survivors in a 64 metre range—1.52 metres less than the biggest possible TR.
However, it may be possible to go even bigger: I'm not sure if it's possible to enter a chase while carrying a Survivor and I'm too lazy to test, but assuming it is:
The Shape with Dead Rabbit addon: Increases EW3 TR by 8 metres.
T3 Distressing: Increases TR by 26%
T3 Agitation: Increases TR when carrying a Survivor by 12 metres.
T3 M&A: Increases TR by 8 metres when in a chase.
So, if it's possible to enter a chase while carrying a Surv:
Dead Rabbit at EW3 Myers, carrying a body and in a chase with all the Perks equipped means 32+8+8+12=60 metres. Add on the 26% Distressing boost, you get 60+15.6=75.6 metres as the largest possible TR in the game.