Yes, LENGTH is a Scrabble word!

Letter values in Scrabble US: L 1, E 1, N 1, G 2, T 1, H 4.

Is it a valid word?

LENGTH is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the linear extent in space from one end to the other; the longest dimension of something that is fixed in place
continuance in time
the property of being the extent of something from beginning to end
size of the gap between two places
a section of something that is long and narrow