this is a worksheet_change macro, which needs to be placed in the
worksheet code area where the action is taking place.
============================
Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
Set Target = ActiveSheet.Range("a1")
If Target > 0 Then
With ActiveSheet.Range("c1:d1").Font
.Name = "Arial"
.Size = 16
End With
Else
With ActiveSheet.Range("c1:d1").Font
.Name = "Arial"
.Size = 10
End With
End If
End Sub
===========================
i didn't know your "default" size, so i chose "10".
c1:d1 is the merged cell.
hope this helps!

susan
On Apr 8, 12:57*pm, MichaelRLan...@gmail.com wrote:
> If the value of A1>0, I need the font size in a merged cell to change
> from the default 10 to 16. *It needs to return to the default size
> when A1 returns to a value of 0. *Can someone help with this?
> Thanks.
>
> Michael