the HFSS manual shows the usage of probe as specified below:
coax outer: vacuum: 0.16mm radius
coax inner: PEC: 0.07mm radius
usually the rule of thumb is to take the inner probe dimension as the half of the outer probe.
In my opinion, your model should consist of the probe dimension as available with you in your lab. for example, at my workplace we have coax outer with diameter of around 1.25mm and then by following the above mentioned rule, you may easily calculate the probe dimension as the half of 1.25mm.
Moreover, the impedance calculations would only give you the position of the feed and not its dimensions at all!!
you basically have to check for the perfect matching point on the patch marked by the Re(Z11) being close to 50 Ohm.
hope it helps.
Ashish