noun
extent
ik-STENT/ɪkˈstent/
How far something reaches or applies.
The measuring tape shows the full extent of the puddle.

Etymology
The underlying image is “stretching out.” Extent names not the stretching itself but how far something reaches, whether measured as distance, range, or degree.
In context
the extent to which others agree with them and underestimate the amount of the extent of differences