Automatically conditionally format font size based on text length

Greetings

I have a staff rota spreadsheet which shows D for rostered days N for nightshift H for holiday and various other abbreviations and I want the cell to automatically change font size for the following conditions:
text length <=2 I want font size to be 11
text length = 3 I want the font size to be 10 and
text length >3 I want the font size to be 5

Being a VBA 'feartie' (I am trying here but its so difficult,dont get me wrong I just love wasting the time messing with VBA but I GIVE UP on this one :-( ) I wonder if anyone might be able to help. I've googled for some ready made code for this and found similar solutions that I have tried to adapt with zero success. What I have learned is:

I need a 'ByVal Target is Range' statement
I need to declare and dimension a variable as a range object which I have called 'cr'
I think a case statement will do the trick.

I've got as far as:

Case (Len(cr.Text)) <= 2
    cr.Font.Size = 11

Case (Len(cr.Text)) = 3
    cr.Font.Size = 10

Case (Len(cr.Text)) > 3
    cr.Font.Size = 5

Case Else
    cr.Font.Size = 11
End Select

Which doesn't create any vba error messages, HOWEVER, when I run the code ... Dan Dan Daaaaaannnnnn!!!!! .... NOTHING Happens

All help, advice, suggestions very gratefully received.

TIA

Rob