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.

$\mathrm{with}\left(\mathrm{Student}\left[\mathrm{LinearAlgebra}\right]\right)\:$

$V≔⟨x\,y\,z\,w⟩$

$V≔\left[\begin{array}{c}x\\ y\\ z\\ w\end{array}\right]$

$\mathrm{Dimension}\left(V\right)$

$4$

$A≔\mathrm{IdentityMatrix}\left(3\,5\right)$

$A≔\left[\begin{array}{ccccc}1& 0& 0& 0& 0\\ 0& 1& 0& 0& 0\\ 0& 0& 1& 0& 0\end{array}\right]$

$\mathrm{rowdim}≔\mathrm{RowDimension}\left(A\right)$

$\mathrm{rowdim}≔3$

$\mathrm{coldim}≔\mathrm{ColumnDimension}\left(A\right)$

$\mathrm{coldim}≔5$

$m,n≔\mathrm{Dimensions}\left(A\right)$

$m,n≔3,5$