VBA To Do 2 Validations On A Cell

On a tab, i have one column where information is entered. what should be entered is based on a dropdown list (which i have setup on a validation). the other thing that should be allowed to be entered is a an 8 digit number. when something else is entered, i need to have a popup message appear to tell the user to fix the error.

I have no idea how to effectively enter in the restriction in VBA for this.

Below is the VBA i have started:

1. The problem is that if one of the drop-downs is chosen, then i get this error message (because the dropdown list has text that is more than 8 characters in length).

2. The one other problem is that if the cell is left blank, i still receive the popup message. how do i avoid this from showing up.

3. I am also looking to do something like this for a range of cells (from F5:F100).

Private Sub Worksheet_Change()

Dim stue As String
stue = "f5"

Application.EnableEvents = True
If Len(Range("f5")) 8 Then
MsgBox "there is an error in cell " & stue & " "
End If
End Sub