Yes, MAGNITUDE is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, G 2, N 1, I 1, T 1, U 1, D 2, E 1.

Is it a valid word?

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

the property of relative size or extent (whether large or small)
a number assigned to the ratio of two quantities; two quantities are of the same order of magnitude if one is less than 10 times as large as the other; the number of magnitudes that the quantities differ is specified to within a power of 10
relative importance

Also: order of magnitude.