Format Dependent on Input Type

You could use a worksheet_change event, assuming these are manual entries:

Private Sub Worksheet_Change(ByVal Target As Range)
    Dim MonitoredRange As Range
    ' change as required
    Set MonitoredRange = Range("A1:B5")

    If Not Intersect(Target, MonitoredRange) Is Nothing Then
        Application.ScreenUpdating = False
        Dim cell As Range
        For Each cell In Intersect(Target, MonitoredRange).Cells
            If IsNumeric(cell.Value2) Then
                With cell
                    .HorizontalAlignment = xlCenter
                    .ShrinkToFit = True

                End With
            Else
                With cell
                    .HorizontalAlignment = xlLeft
                    .ShrinkToFit = False
                End With
            End If
        Next cell
    End If

End Sub