The Size(M) function returns an array with the number of rows as the first entry, and the number of columns as the second entry.

When M is a row-vector, the result of Size(M) will have a 1 in the entry specifying the number of rows.  Similarly when M is a column-vector, there will be a 1 in the second entry of the result.

When M has more than 2 dimensions, the number of entries in M will correspond to the number of dimensions of M.

When n is given, Size(M,n) will return only the number corresponding to that dimension.  Size(M,1) will give the number of rows and Size(M,2) will give the number of columns.

When n is larger than the number of dimensions of M, 1 will be returned.

When there are multiple return values as directed by a sequence of names on the left side of :=, each variable will match with the corresponding dimension size.  If there are fewer variables than dimensions, the last variable will be assigned the product of all remaining dimension sizes.

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

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

$M≔\mathrm{Matrix}\left(2\,3\right)\:$

$\mathrm{Size}\left(M\right)$

$\left[\begin{array}{cc}2& 3\end{array}\right]$

$\mathrm{Size}\left(M\,2\right)$

$3$

$m,n≔\mathrm{Size}\left(M\right)\:$

$m$

$2$

$n$

$3$