P.S. There is a completely different word, "extant." It has nothing at all to do with "extent." "Extant" is fairly rare and means "still in existence."

It is a fairly formal word. Literally, it means the complete, total, largest distance or area covered by something. "A landslide in California has blocked the Pacific Coast highway." "What's the extent of the landslide?" "400 meters of the highway are covered." "The Roman Empire reached its greatest extent in 117 CE." In 117 CE, the total amount of territory controlled by the Roman empire was at its peak. Figuratively, it can mean "the full measure of something." Michael Chambers' examples illustrate this. In computers, the word "extents" is used to describe certain file systems. A file's index entry on a disk may be a list of "extents." Each extent is a set of consecutive disk blocks that the file uses. For example, a single file might be stored on the disk as three "extents," blocks 156-172, blocks 315-321, and blocks 483-509.

e.g. I agree with you on this issue to some extent / a great extent = I partially agree with you on this issue You can use it in lots of contexts. It is formal but still common.