Yes, DIMENSIONS is a Scrabble word!

Letter values in Scrabble US: D 2, I 1, M 3, E 1, N 1, S 1, I 1, O 1, N 1, S 1.

Is it a valid word?

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

the magnitude of something in a particular direction (especially length or width or height)
a construct whereby objects or individuals can be distinguished
one of three Cartesian coordinates that determine a position in space
magnitude or extent