Correct spelling, explanation: suffix -th used with abstract adjectives makes a noun describing the condition of something. The correct form length is thus a combination of long and this suffix -th. This is also why lenght is an incorrect form.

Definition of length:
noun, longer or longest measure of something
Length of this table is enough for us, but I’m afraid it’s too narrow.

Idioms and expressions with length:
at length, fully, thoroughly; at last:
The project was made at length; it was nearly perfect.
to go to any lengths, when someone is very desperate and wants to achieve something no matter how extreme their methods are:
I don’t understand people who go to any lengths to get a job – they would even be dishonest!
at arm’s length, to keep at bay or to be very close to a person (so you can reach it with your arm straight):
Keep her at arm’s length. She’s a gossip.
I always put my phone on my bedside table to have it at arm’s length.

Incorrect spelling, explanation: lenght is the wrong spelling because to create a noun from an adjective long we need to use suffix -th, which gives the meaning of quality. Therefore the correct spelling will be length. E remained in this noun from Old English form: lengthu. With ending ht the form lenght is incorrect.

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling