If you enter 'spare wheel well' in the search field at the top of the page you will find several previous threads on this, which also discuss other things you might need to consider , such as a jack, wheel brace and the special centre bolt to clamp the wheel down in the well. But certainly worth the effort in view of the increase in potholes.
IIRC the consensus is a full sized spare should fit in the well of a mk2 and some mk3's. But at some point during mk3 production the depth of the well may have been reduced slightly. Deep enough for a space saver, but a full sized spare might protrude slightly so the boot floor is no longer completely flat. But not by much, and its not difficult to correct using a false floor of hardboard, thin ply or even cardboard. Wheel circumference/diameter is never a problem (except in the mk4 which as no spare wheel well at all ) .
