If you think about it, 4.2 inches is a bit of a strange size for a LCD screen. In decimal, 4.2 inches 'translates' as 106.68mm, which isn't an even number either. So why did Nintendo decide on 4.2 inch screens for the DSi XL. The answer, it seems, lies in the manufacturing process.
With LCD screens, the cost of each screen is found by calculating the amount of screens you can cut from one large LCD sheet. Nintendo worked out that cutting a screen size of 4.2 inches or 106.68 mm was most efficient, producing the least amount of waste. This keeps the price down and the environmentalists off your back!
Producing a larger screen with the same pixel resolution could result in a grainy image and this was a real concern for a number of Nintendo employees. To put their minds at ease the LCD manufacturer came in to demo the screens and to everyone's relief there was no noticeable drop in image quality.
When the DS Lite was produced a few years ago, wide viewing angle LCD's were far more expensive than conventional LCD's. Although they considered these LCD's for the DS Lite, they ruled them out because of the excessive cost. Fast forward a few years and the price differential has narrowed to the point where it made sense to use the wide-viewing angle LCD's. As a result of installing these screens, the DSi XL screen is clearly visible even when viewed from the side.