Select from Cell A1 until the Last Cell

Dim rLastColumnLetter As String

Dim rLastRowNumber As Long

rLastColumnLetter = Split(Columns(Range("A1").End(xlToRight).Column).Address(, False), ":")(1)

rLastRowNumber = Range("D" & Rows.Count).End(xlUp).Row

Select from cell A1 until the last cell.

Range("A1:" & rLastColumnLetter & rLastRowNumber)