Checking Field Length

I'm typing into an unbound textbox.
I want to limit the number of characters that can be entered.
As your typing the letters in, if you go over a certain number, lets say 5
I 
want a msgbox (that Imade to pop up)
I tried this on the after update event but it doesn't seem to fire until 
after one leaves the field.  I need it to check in live time.  Any help is

appreciated.
Thanks
DS

Dim SZ As Long
SZ = Nz(Len(Me.Text0), 0)
If SZ > 5 Then
    DoCmd.OpenForm "frmMsgInfo"
    Forms!frmMsgInfo!TxtMsg = "5 Characters Only"
Else
End If