Private Sub UserForm_load()
Dim menor As Double
Dim mayor As Double
Dim a As Double

menor = 1.79E+308
mayor = -1.79E+308

comienza:
If a = 99999 Then
    MsgBox menor & " " & mayor
Else
    a = InputBox("Numero")
    If a < menor Then menor = a
    If a > mayor Then mayor = a
    GoTo comienza
End If
End Sub