In VBA, you can use Application.Caller.Rows.Count to find out the expected size of an array forula, so you can return an array of the right size.

In c# you can also do this!

Check out the page Writing an Excel COM Addin first so you can get hold of the Application object.

Then:

Excel.Range range = (Excel.Range) application.get_Caller(System.Type.Missing);
			
int rows = range.Rows.Count;
int cols = range.Columns.Count;