Dimensions

Dimensions[expr] gives a list of the dimensions of expr.

Examples

Dimensions of a matrix:

Dimensions[{{1, 2, 3}, {4, 5, 6}}]
(* {2, 3} *)

Dimensions of a vector:

Dimensions[{a, b, c, d}]
(* {4} *)

Please visit the official Wolfram Language Reference for more details.