Hi Nal,
PTA5 is input only on a QD4. RTFM, that's what I just did!
To determine the pullup resistor value:
Enable the pullup on an input pin.
Temporarily connect different value external pulldown resistors.
Measure the pin voltage at the same time.
When you get 1/2 Vdd then the external resistor equals the internal one.
From memory on other S08's the value is something like 40k.