Wow that got a bit complicated...
So, it would end up like,
Dim StartDate As Variant
StartDate = Nz(Me.[TextBox Startdate], 0)
If IsNull(StartDate) then Exit Sub
ElseIf Me.[TextBox Projectsize] = "Small" Then
Me.[TextBox Enddate]= DateAdd("d",20,StartDate)
ElseIf Me.[TextBox Projectsize]...