Excel-VBA: How to get size of selected array in vba code?

Pass a Range object into your function and look at the value:

Public Function WhatIsTheMatrix(SourceRange as Excel.Range) As Variant

   Dim arrSource as Variant
   Dim arrOutPut as Variant

   Dim i As LongDim j As Long

   arrSource = SourceRange.Value2

   i = Ubound(arrSource, 1)
   j = Ubound(arrSource, 2)

   ' The lower bounds of an array obtained from a range are always 1

   ' code
   ' more code
   ' even more code

   WhatIsTheMatrix = arrOutPut 

End Function

Now you have two problems:

1. Your original problem, which is solved by inspecting i and j

2. A special case, in which the value of a single-cell range isn't an array variant

So you'll need this:

If SourceRange.Cells.Count = 1 Then
   Redim arrSource(1 to 1, 1 To 1)
   arrSource(1,1) = SourceRange.Value2
Else
   arrSource = SourceRange.Value2
End If

Any other Business:

There's an unrecognized assumption, namely: you thought you could pass in the current Application.Selection and it will always be a range. It can also be a control, or a chart, or a chart series...

...And no, you can't rely on this raising a runtime error for the type mismatch. Yes, you've invoked Option Explicit and yes, you typed SourceRange as Excel.Range and your Mother, your Parish Priest and a Justice of the Peace watched you do it; try sacrificing something on a sinister-looking rock after midnight, and maybe the VBA runtime will believe you.

So you also need this:

If Not TypeOf SourceRange Is Excel.Range Then
   ' Raise an error or Exit Sub
End If

I think that's all the surprises. Unless you're into exotic stuff and awkward questions like "Is this range calculated?"