Size matters, or does it? Well it might depend on your particular area of expertise, but in the world of mobile consumer technology, small is beautiful and there are reasons why things are the size they are.

Mostly it is technical limitations. Some internal components, batteries antenna etc, are hard to shrink much further. Others – high definition touch screens, slim-line laptop cases – may not be as cost-effectively produced in larger sizes because of increased failure rates in the manufacturing process.

There are other things that dictate size. Mobile phones need to be small enough to be comfortably held to the side of the head and fit snugly into pockets or handbags, and tablet devices that demand more interaction need to be as large as possible to allow for more sophisticated interaction, yet still sufficiently small and light to handle anywhere.

Occasionally some sizes have been thought to be ‘just right’. However, this ‘Goldilocks’ principle that guided Apple under Steve Jobs to stay with the 3.5-inch iPhone and the 10-inch tablet seems to be under review – at least if the rumours concerning Apple’s new products come true.

But if 3.5-inch is such a good size for a mobile phone, then why change? There is certainly competitive pressure for a bigger screen, with many of the other manufacturers tackling Apple’s hold on the market by ‘going large’. The thinking appears to be something along these lines: users want to do more while mobile, so let’s give them more space.

The downside to this approach is two-fold: hand size and pocket size. Neither is easily upgradeable, although many clothes manufacturers have done a sterling job of keeping up with the varying sizes of mobile phones and smartphones. The Galaxy Note, for instance, will fit in most people’s pockets these days, and that’s essentially a small tablet.

If the rumours are true, Apple is making the next iPhone taller, but not wider. This might avoid the comfort issues of wider phones in small hands and fit from a fashion perspective – but why bother? Taller might be better for widescreen 16:9 video viewing, when turned on the side, but this is just one application of the device – many other applications would need to be changed before they could take advantage of the extra screen space.

Developers might love the target rich (i.e. large and free spending) environment of the Apple user base, but they’ve had to think about iPads and high definition retina displays in recent years, so presenting them with yet another screen size would be a tall order.

However using new screen space for dedicated soft functions or controls might be of interest, especially if it aids usability. The home button could move there and save a hardware component and it could be used for app navigation or advertising. It might even be used for new functionality that needs to be visible without interrupting the main app on screen – like a payment system…

The taller screen is also expected to be built in a taller overall package and a bit more height might also allow Apple to address another thorny issue, limited storage – of both power and memory.

As a larger device there should be more room for a bigger battery and, perhaps based on a recently publicised Apple patent, some detachable storage – that means SD-support. The addition of a new smaller charger/data connection, which many reports claim is now a dead-cert, would also, theoretically, create enough room for a card slot too.

Height isn’t likely to be the only element to change in the iPhone 5’s external appearance. There has been talk of using different materials in the case and backplate, in particular the potential use of ‘Liquidmetal’ technology – a strong metal alloy that is easily shaped.

If Apple changes the external connector, this might be a good time to change the shape of the device as well. Liquidmetal would allow for the inclusion of a curved back plate. With nearly all smartphone manufacturers having followed the square slab with rounded corners look, it might now be the time for Apple to throw in a curve ball, or at least, curved back?

The prospects for a size change in the iPad are less likely than those with the iPhone, but not because of some Cupertino inspired idealism. While many would welcome a low cost iPad (and therefore a ‘Nano’ tablet device might fit the bill), it might look too much like a follower to Amazon’s Kindle, and less like the universal tablet Apple has created in a market it currently dominates.

In addition shrinking the iPad does not fit with Apple’s focus on usable design, and it is difficult to envisage where an iPad Nano might fit alongside the iPhone – it would deliver too little interaction to be a great tablet and be too big to be pocketed like a phone.

Competing head to head with other e-book readers might be a perfectly acceptable position for many vendors, but unless it can make a radical change, entering an established market with a ‘me too’ device does not really sound like something Apple likes to do.

Perhaps as some of the cheeky blogs commented in 2010, the best iPad Nano might turn out to be the new iPhone?