I’ve just spent the last three days pottering around in the new Saab 9-5. This is one big machine, just nudging over five metres from nose to tail. It’s very wide, too.

The six months I spent in the related Vauxhall Insignia were marked by the difficultly of squeezing into a typical parking space and then trying to get out when the door could only be opened by a few inches.

The 9-5 is based on the long-wheelbase version of GM’s Global Epsilon platform (aimed mainly at the legroom-obsessed Chinese market), which makes it hard to fit into the parking spaces lengthways as well as widthways.

Turning into side roads also takes a bit of extra care because the 9-5 is front-drive. Rear-wheel drive makes it easier to shuffle a long car around the city.

There’s no doubt that the sense of space in the 9-5 cockpit is life-enhancing, but this new Saab must also mark the high point for the ever-expanding girth of mainstream cars.

For years we have got used to cars getting bigger with every generation. But there’s good reason to believe that this expansion has reached its zenith.

Firstly, people are unlikely to get much bigger. The difference in size between the average male in 1945 and today is quite marked. But that spurt of nutrition-fed growth is coming to an end.

And the link between vehicle size and refinement has been broken. Today’s Golf is quiet and refined enough for easy transcontinental travel.

Space is also increasingly at a premium in urban centres and set to get more precious as more people move into the cities in much of the world, so mainstream cars cannot get much bigger.

A senior BMW engineer recently told me that the new 5-series was aimed at buyers in their fifties - one of the reasons it was a less hardcore driving machine.

According to most car makers, in fact, over the next couple of decades the older, size-loving, generation will fade out of the new car market to be replaced by the coming generations who favour handiness over sheer sheet-metal real estate. And so the average size of the average car will start to shrink.

Meanwhile, if you think this 9-5 looks big, wait until you see the estate version…