in an L&K mk1 octavia, with the "superior sound system" jazz, what size are the rear speakers?

6.5" or 5.25" or some weird other size?

and anyone know the mounting depth in there? looking at some speakers at 6.5" diameter, 84mm depth. will they fit?

estate or hatchback? IIRC estate has weird shape and size, not sure about the hatchback.

You could always save the kitten and try a search, there are enough threads on speakers.