Sub SelectAllCellsInSheet(SheetName As String)
lastCol = Sheets(SheetName).Range("a1").End(xlToRight).Column
Lastrow = Sheets(SheetName).Cells(1, 1).End(xlDown).Row
Sheets(SheetName).Range("A2", Sheets(SheetName).Cells(Lastrow, lastCol)).Select
End Sub
To use with ActiveSheet:
Call SelectAllCellsInSheet(ActiveSheet.Name)