You spelled it correctly: extent.
Example:
The extent of the flood's damage was not seen until daylight.
The adverb meaning to a great extent is very.
The likely word is "least" (of the smallest quantity or extent).
The likely word is "diagnose" (to determine cause or extent, as of illness).
The correct spelling is "extremely" (to a great extent).
The correct spelling of the adverb is mostly (to a great extent, primarily).
The correct spelling of the adverb is "especially" (to a great extent, or specifically).
That is the correct spelling of "moreso" (to a greater extent).
The correct spelling of the adverb is "extremely" (very, to a great extent or degree).
That is the correct spelling of the adverb "slightly" (to a small or minor degree or extent, somewhat).
length, time, period, term, stretch, extent, spell, span
The likely word is the adverb highly (to a great extent, to a height).
The correct spelling is "length." It refers to the measurement or extent of something from end to end.