I take your point about the importance of layout completelyOur current MH is a bit less than 7.5m, though we usually have a bike rack on the back so that takes us up to just over 8m overall. We've only been able to use it for about 3 weeks in total so far, but we haven't had any problems yet. Our previous MH including the bike rack was about 7.25m and that wasn't any easier or more difficult than the current one - so the extra 75cm length has had no impact so far on where we can go. Roads have hardly ever been a problem (though we tend to steer clear of the smaller ones if possible), but we have occasionally come across campsites or stops which have been pretty tight for manoeuvring, even with the slightly smaller MH. We tend to stay on a mix of campsites, aires and France Passions, and where possible I tend to scope out the roads and access on Google maps if I can to double check, but you can't always spot that the pitches are tight.
In terms of useable inside space though that extra 75cm makes a huge difference to us, though some of that is also due to a slight change in layout. I think what I'm trying to say is that its not just about the overall size of the vehicle, but also about the layout.....I remember looking at a 6m Pilote a-class which due to the layout had a huge amount of space inside.