Hi Paul
We're just back from 3 weeks in Italy with our single-axle Bailey Senator Arizona (7.20 metres). Last year we took our twin axle Avondale Landranger (7.36 m).
A good source of info is the Caravan Club "Caravan Europe" site guide(cheaper from Amazon than the Caravan Club members' Special price!!) As well individual site reviews/details it has a wealth of info on each country and its laws and a seperate section detailing Mountain passes and tunnels and any restrictions. There are 2 volumes covering - (1) France, Spain Portugal and Andorra (2) The other Euorpean countries,including Italy. To quote from the latter volume for Italy. "Acarvan with one axle should not be longer than 6.5 metres, or with 2 axles, 8 metre, but again tourists are exempt from rule."
I would, also, advise you that Italian campsite pitches do not tend to be very large, presumably because of their rules on 'van size". On the other hand we have always managed. Both last year and this we have secured suitable pitches by phoning ahead on the day of arrival or the day before. Our last pitch in at Riva San Pietro on Lake Iseo was 7.5 metres square so quite tight on size as there were no spaces between pitches. Cars were parked seperately nearby.
Italy is a beautiful country - Go for it would be my advice.
Regards
Tim