Hello Heinz,
I saw the replies to your post on the Yahoo Sidewinder Group and know you'll get different opinions as always......
Have now flown my Sidewinder for over six years and have almost 1000 flights logged. I think the answer to your question re. size is very much an individual thing. It is certainly 'cozy' especially with both seats filled, but not uncomfortable for people of average size. I am just over 181cm and 88kg and fit ok but perhaps like some other builders, I made the aircraft (seat position, rudder pedals, controls etc.) to fit me. I therefore think that if you have a particular aircraft in mind to buy, it would be best to try it on rather than relying on what other folks have to say about it.
As a guide though, you could seat yourself and your wife (or flying partner) on side-by-side chairs and measure the outside-shoulder distance. In the Sidewinder the seats are close together and the distance between the canopy rails, if built per plans, is 87.63cm.
Chris