You have to Dim the array  to start with and then ReDim it afterward.
There are some caveats to using Redim though so check the help. If
it's a one dimension array you should have no problems.

The code  should look something like

Dim slArray() as string
Dim ilArray as integer
..
..
ilArray = 20
ReDim Preserve slArray(ilArray)

.. or

ReDim Preserve slArray(Ubound(slArray)+1)