Thanks to Eddypeck for his time and effort.
Standard speaker sizes are:
In dash 3.5" (if door speaker are fitted this is a 1" dome tweeter in a 3.5" housing)
In front doors 5.25" (if fitted)
In rear parcel shelf supports 6"x4"
US Spec had 5"x7" in the rear parcel shelf supports as the rear seat belts are only lap belts so no inertia reel in the way.
See here: http://www.vwgolfmk2.co.uk/forum/viewtopic.php?f=137&t=47382
For further information see Audio section: http://www.vwgolfmk2.co.uk/forum/viewtopic.php?f=137&t=49298
the links above are dead.
i've bluntly put two 4 ohm visaton fr8 (https://www.visaton.de/en/products/drivers/fullrange-systems/fr-8-4-ohm) into the dashboard.
they matched the holes but i needed to clamp them with the screwedges, cause short latches.
for the pins i made adapters using cable lugs, so i do not have to change the wiring of the car.