public elements()
public fin_len
' ...
sub recurs(cur_substr, cur_pos)
if ubound(elements)-len(cur_substr) < fin_len-cur_pos then exit sub 
if len(cur_substr)=fin_len then then print cur_substr:exit sub 
for i = cur_pos+1 to ubound(elements)
  call recurs(cur_substr & elements(i), i) 
next i
end sub
' ...
call recurs("", 0)
' ...

Public elements() As String
Public fin_len As Integer

Sub recurs(cur_substr As String, cur_pos As Integer)
Dim i As Integer
If UBound(elements) - Len(cur_substr) < fin_len - cur_pos Then
    Exit Sub
End If
If Len(cur_substr) = fin_len Then
    'Debug.Print cur_substr
    Exit Sub
End If
For i = cur_pos + 1 To UBound(elements)
  Call recurs(cur_substr & elements(i), i)
Next i
End Sub

Sub test()
Dim i As Integer, cnt As Integer
For cnt = 11 To 30
    ReDim elements(1 To cnt)
    For i = 1 To cnt
        elements(i) = "1"
    Next i
    fin_len = 10
    t = Timer
    Call recurs("", 0)
    Debug.Print cnt, fin_len, Timer - t
Next cnt
End Sub