Wheelnut has cheated slightly by parking on grass
hiding the tyre sidewall height!
It is, though, a lovely photo - what a great colour - really suits the Spider's design period. Metallic red (Proteo) is the same era but I like that green more - the red is similar to a BMW colour (Calypsorot) whereas the green is more unusual.
Also, Wheelnut, can you please confirm the tyre size - 205/50 16 as Clive said for the GTV? They definitely look lower-profile than the tyres I've seen on 156s.
My Spider's on 17s and I agree with Clive that 16s give a nicer ride (and are a lot more common/cheaper to find) - my last Spider had 16s and seemed to soak up small ripples/rough surfaces better. The weight of the wheel/tyre combo is important too. Less unsprung weight=better ride in my opinion.
At the end of the day though, the best wheels are the ones in good condition. Changing wheel size can be quite a cost if you have to fork out for four new tyres - the used tyres, even if nearly new, tend not to recover their cost if you try and sell.
Also bear in mind that new tyres of good quality will also have improved smoothness compared to old tyres. My Spider has two old (original version) Eagle F1s on the back, and the noise/roughness made me think a wheel bearing had failed (I've checked the bearings and they're fine). I understand that new Eagle F1s have been improved but even so, just a warning that tyres can make a difference as well. Confusing!