This "length not exceeding" thing is another way of getting more money - you will notice many ferries use whole number metres, so a 7.1m van gets charged as 8.
What they dont do is to credit you any lenght remaining for the car - you usually pay for "up to 5m"
I maintain you should be able to use the full footprint you pay for, such that if you pay for 5m car and 7m van, then you should get 12m space, such that 7.2mvan and 4.8m car (including towing bracket, to be fair) should be OK.
Similarly with height suppliments. If you are not careful and have a tall 4x4 then you may pay the height suppliment twice. The CC site specifically advises about this when booking for Brittany ferries.
Travelling mainly off peak, i have never had a problem, but there's always a first time.