Assuming you are flying Qantas ... For international flights. exit row seats can be pre-allocated by QF Platinum FF members. Since you are not a Platinum FF member, your best option is to turn up very early for check-in and ask if there are any exit row sats remaining.
Note that check-in usually opens about 3 hours before departure for QF international flights. But if you have a domestic connection prior to your international flight, you will be checked-in for the international sector at the same time as you check-in for the domestic flight. In this way its possible that many people have had the opportunity to request an exit row seat long before the flight opens for check-in at the departure airport.
You can also call Qantas and ask them to make a note in your booking info that you are very tall and request a seat with extra leg-room. They will make it very clear that there is no guarantee that this will be honoured, but it can't hurt to ask to have the info added to the PNR details.
Don't even think about an operational upgrade as a bronze FF member flying with Qantas.
If you have a relative who is a Qantas Platinum FF member you can transfer QF FF points to their account and ask them to waitlist you for an upgrade using the points from their account. That way you get to the top of the upgrade waitlist due to the status of the member requesting the upgrade. Depending on how busy the flight is, this is probably your best chance for an upgrade, but if it comes through the waitlist process you will be parting with a sizable number of FF points.