With the Build-In function a change of the detection radius is not possible as far as I know.
Many users use additional geofence apps on their smartphones, partly because the build-in detection is (was?) not reliable.
With the app Owntracks you can define the radius yourself. You can find an instructions here.
I also have the app Geofency in use, as @Shiki already suggested and I’m satisfied. But the minimum detection range via geofence is 50 meters with this app. However, I’m not sure if a detection radius via geofence of < 50 meters makes sense, because the accuracy is usually higher, especially when there is movement.
Edit: @Shiki, there is no need for the Webhook Manager. It works with the logic card “A WebHook was received” in the “When…” section and the logic card “>Tag< is exact” in the “And…” section.