The Dimension(A) command, where A is a Vector, returns the number of elements in A.  If A is a Matrix, two non-negative integers representing the row dimension and the column dimension of A, respectively, are returned.

Dimensions(A) is an alternate form for Dimension(A).

The ColumnDimension(A) command, where A is a Matrix, returns the number of columns in A.

The RowDimension(A) command, where A is a Matrix, returns the number of rows in A.