The Dimension(A) function, where A is a Vector, returns a non-negative integer that represents 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 RowDimension(A) function, where A is a Matrix, returns a non-negative integer that represents the number of rows in A.

The ColumnDimension(A) function, where A is a Matrix, returns a non-negative integer that represents the number of columns in A.

This function is part of the LinearAlgebra package, and so it can be used in the form Dimension(..) only after executing the command with(LinearAlgebra). However, it can always be accessed through the long form of the command by using LinearAlgebra[Dimension](..).

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

$V≔⟨x\,y\,z\,w⟩$

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

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

$4$

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

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

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

$\mathrm{rowdim}≔3$

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

$\mathrm{coldim}≔5$

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

$m,n≔3,5$