Hey All
I'm looking to do the same as the OP Nutter244. My little plain Jane 2.0 GLX hatch has 165/170 in the front doors and that's it! I have 2 speakers! I gracefully (fat guy remember) crawled inside my boot and measured the mounts for rear speakers, I have the mounts but no speakers. They appeared to be 100mm/4inch. It may be an idea to double check the mounting points in your own variant of Octavia.
Side not: I have recently switched from FM/AM to a DAB unit with fantastic results, during the switch over I noticed that I don't even have wiring for rear speakers!! Simple plug and play operation has become a pulling out interior trim and running wires operation. Again, double check you have everything needed in place so you know exactly what is required for the job.