I like to used this method the most, it will auto select the first column to the last column being used. However, if the last cell in the first row or the last cell in the first column are empty, this code will not calculate properly. Check the link for other methods to dynamically select cell range.
Sub DynamicRange()
'Best used when first column has value on last row and first row has a value in the last column
Dim sht As Worksheet
Dim LastRow As Long
Dim LastColumn As Long
Dim StartCell As Range
Set sht = Worksheets("Sheet1")
Set StartCell = Range("A1")
'Find Last Row and Column
LastRow = sht.Cells(sht.Rows.Count, StartCell.Column).End(xlUp).Row
LastColumn = sht.Cells(StartCell.Row, sht.Columns.Count).End(xlToLeft).Column
'Select Range
sht.Range(StartCell, sht.Cells(LastRow, LastColumn)).Select
End Sub