AUTOFILL IN EXCEL MACRO - vba

Question

I am trying to use macro recorder in Excel to record a macro to fill down a column of cells, however because the fill down each time is a different number of cells it either fills down to short or too long and this seems to be because the macro identifies the cell range and its fixed.
What I need is to auto populate or auto fill down from:
ActiveCell.FormulaR1C1 = "=CONCATENATE(RC[1],"" "",RC[2])"
Range("C1").Select
Selection.AutoFill Destination:=ActiveCell.Range("A1:A261")
ActiveCell.Range("A1:A261").Select
Since file is not always 261? How can I place a command to choose/autofill the last column?
Blockquote
Sub WMEHOT_Cleaner()
'
' WMEHOT_Cleaner Macro
'
'
Columns("A:A").Select
Selection.Insert Shift:=xlToRight, CopyOrigin:=xlFormatFromLeftOrAbove
Selection.Insert Shift:=xlToRight, CopyOrigin:=xlFormatFromLeftOrAbove
Columns("L:L").Select
Selection.Cut
Columns("B:B").Select
ActiveSheet.Paste
Columns("C:C").Select
Selection.Cut
Range("O1").Select
ActiveSheet.Paste
Range("C1").Select
ActiveCell.FormulaR1C1 = "=CONCATENATE(RC[1],"" "",RC[2])"
Range("C1").Select
Selection.AutoFill Destination:=ActiveCell.Range("A1:A261")
ActiveCell.Range("A1:A261").Select
Range("C1").Select
Range(Selection, Selection.End(xlDown)).Select
Selection.Copy
Selection.End(xlUp).Select
Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks _
:=False, Transpose:=False
Application.CutCopyMode = False
Columns("D:E").Select
Selection.ClearContents
Range("D1").Select
ActiveCell.FormulaR1C1 = "=CONCATENATE(RC[2],"" "",RC[6])"
Range("D1").Select
ActiveCell.FormulaR1C1 = "=CONCATENATE(RC[5],""_"",RC[6])"
Range("D1").Select
Selection.AutoFill Destination:=ActiveCell.Range("A1:A261")
ActiveCell.Range("A1:A261").Select
ActiveCell.Select
Range(Selection, Selection.End(xlDown)).Select
Selection.Copy
Selection.End(xlUp).Select
Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks _
:=False, Transpose:=False
Application.CutCopyMode = False
Columns("I:J").Select
Selection.ClearContents
Columns("O:O").Select
Selection.Cut
Columns("E:E").Select
ActiveSheet.Paste
Columns("F:F").Select
Selection.Insert Shift:=xlToRight, CopyOrigin:=xlFormatFromLeftOrAbove
Columns("L:L").Select
Selection.Cut
Columns("J:J").Select
ActiveSheet.Paste
Range("J1").Select
Selection.Copy
Range("L1").Select
ActiveSheet.Paste
Application.CutCopyMode = False
Range("A1").Select
ActiveCell.FormulaR1C1 = "=""WMEOnline_""&RC[9]"
Range("A1").Select
Selection.AutoFill Destination:=ActiveCell.Range("A1:A261")
ActiveCell.Range("A1:A261").Select
ActiveCell.Select
Range(Selection, Selection.End(xlDown)).Select
Selection.Copy
Selection.End(xlUp).Select
Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks _
:=False, Transpose:=False
Application.CutCopyMode = False
ActiveCell.Offset(2, 0).Range("A1").Select
Range("O5").Select
Cells.Select
Range("A1").Activate
Selection.Copy
Workbooks.Add
ActiveSheet.Paste
msgclean = MsgBox("Cleaning and Sorting Complete!!" & vbNewLine & "File Ready for LMS." & vbNewLine & "Please SAVE this file as CSV Format", vbInformation + vbOKOnly, "WME HOT Cleaner Template")
End Sub

Answer 1

Use the following code:
Range("A1:A" & Range("A" & Rows.Count).End(xlUp).Row).Formula = "=CONCATENATE(RC[1],"" "",RC[2])"
Replace A if your column is not column A

Answer 2

Use
lr = Cells(Rows.Count, "A").End(xlUp).Row
to find the last row from a column or
Set wks = ActiveWorkbook.Worksheets(sheet)
i = wks.Range("A:A").End(xlDown).Row
j = wks.Cells.End(xlToRight).Column
to find the position of the last cell.