If the crew of the first Shuttle arriving in space bumped into some Martians, and assuming the crews were predominantly male, talk would quickly turn to comparing rocket sizes and the specifications and relative performances of their craft.

If the Martians asked the embarrassing question, “How did you define the optimum size of your engines?”, the correct and honest response would be, “The size of the engine is as wide as two horses’ backsides”.

The conversation would have then probably become Monty Pythonesque. “What’s a horse?”, and on from there. You get the picture.

But there’s some truth to this. The engines of the Shuttle were manufactured in another part of the US and had to be carried to the assembly site by train as they were too big for air or road haulage.

The train line passed through a tunnel, and the size of the tunnel dictated the maximum size of the engines. The train track gauge was defined by the width of the horse-drawn carriages it replaced, and the width of the track was defined by the fact that the wheels had to run behind the horses’ feet as this is where the best, or only, road was.

So, the past often dictates the future of the market. In the case of mobile phones and tablets, we are dealing with similar legacy and physical limitations.

In the case of the mobile phone, the physical constraints are the size of your hand and the distance from your ear to your mouth. We could also talk about the hands-free Bluetooth headsets, but this requires a behavioural shift and history has shown that if we are given the choice of behavioural change or adjusting a device to meet our needs, the latter will always be the bigger market.

So, from an ergonomic point of view, there has to be an optimal size for these smart devices.

We have to create a device that has the largest screen, with the right size of icons, which falls in line the constraints above.

A smartphone has to fit in a pocket and not have to be squeezed in. The fashion world isn’t about to match your apparel to a bigger device and you’re certainly not going to replace your wardrobe every time you get a new phone. We have to assume average pocket sizes will remain relatively unchanged.

Pocket size is relative to the size of the clothing, so all pockets are not the same. If you are a 150kg, pie-eating man in the US, then you’ll have big pockets. If you are from the East, chances are your pockets will be smaller. Women don’t usually have pockets, at least in the work environment. They tend to carry their phones or have them in their handbags. They have justified their fairer-sex label — they are more conscious of their appearance and so are less likely to walk around with a dinner plate stuck to the side of their face trying to look intelligent.

They may not have the same pocket-sized constraints of men, but the device market is now a truly global market, so devices are produced for mass consumption, taking into account all of the above constraints.

Samsung has just raced past Apple as the leading smartphone maker worldwide. The difference here is the Korean company has done it with a range of device sizes and Apple has done it with just one. Samsung has a phone for everybody, but has a couple of particularly strong models. As the market leader, we have to assume it has some learnings and the new Galaxy S3 is right for the market. It has a large, 4,8-inch screen. The new iPhone will, in all likelihood, have a 4-inch screen.

The tablet market has similar but less stringent constraints. The upper size limit has to be the current PC screen size. Why go bigger? The lower screen size has to be the size of the average novel. Anything smaller will put you into the oversized phone market, which is limited.

The optimum size would, of course, be A4: that is what we are used to. Our filing cabinets, drawers, briefcases, shelves and desks all have real estate allocated to cater for A4 sizes. The origin of the humble A4 is in pre-revolutionary France, when the metric system arrived. A0 is a square meter, if you fold it in half you get an A1 and so forth. So it has logical origins and we have built our working world to accommodate it.

This covers most magazines, too. Few have established niches in the smaller, A5 format. Most are A4 or similarly sized. The other logical format is the size of a novel. That is what we feel comfortable with when reading, in bed, on the train or on the plane.

History suggests we’ll end up with one phone size, with a touch screen between 3,2 inches and 4 inches in size as the market norm. It will go thinner and thicker, as we make it light for portability and heavy for processing and battery life, but the real estate will stay roughly the same.

The tablet market will probably end up with two sizes, one just under A4 and one novel-sized. The A4 camp will be just under A4 size as we tend to put covers on them and they still need to fit where we need them to fit.

This is, of course, all informed speculation but as a species we tend to avoid change. If we can mitigate the impact of change in our lives we will go to great lengths to do it.