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 correct spelling of the adverb is "especially" (to a great extent, or specifically).
The correct spelling of the adverb is mostly (to a great extent, primarily).
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 "extremely" (very, to a great extent or degree).
That is the correct spelling of "moreso" (to a greater extent).
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.