It seems to me that the article assumes that a 15" tablet would be most optimal because most laptops sold come with a 15" screen size. But it doesn't really dig deep when it comes to WHY people buy 15" laptops.
I did some research on buying a laptop recently and it struck me that if you're on a budget or you want a certain degree of good performance, especially when it comes to gaming, 15" is the way, sometimes the ONLY way, to go. For Windows based laptops, price jumps up by quite a lot when screen size goes down below 15" if you want to maintain performance.
It's also worth considering - How many of those 15" laptops are actually desktop replacements that never leave the house? Laptops used to be horribly expensive, but 15" laptops are quite reasonable these days and most people don't need more power than what a 15" laptop can provide. It doesn't take up much room and you can move it from the office, to the kitchen and the living room as desired. But how many people actually bring these devices with them when going outside? I see plenty of people with 11" and 13" laptops during my commute, but rarely 15" laptops.
But all that doesn't apply to tablets. It's like saying that because apples are round, so too should bananas. Have fun peeling a round banana
The thing is, the way we use tablets is completely different from how we use laptops. We hold tablets closer to our face and we like to carry them around, even using them while standing up. You just can't compare the two and say because one size is the most popular, the other device should have the same size. We use our devices for different purposes. My phone fits in the pocket of my pants. My iPad Mini 2 fits in the pocket of my jacket. Great for longer trips. My iPad 4 sits on my desk as a secondary screen when using my desktop or serves as a laptop "replacement" when I want to consume media while being social with my wife. When I want to lean back and read though, my iPad mini comes back into service.
A 15" tablet could possibly replace my regular iPad, but it couldn't replace my phone, mini or desktop. If I had a laptop, maybe it could replace that. But what I really would like a 15" tablet for is creating digital art. That's what intrigues me about the Surface Pro 3. There are some things where the bigger screen format could shine, just like there are some things where the small screen size comes in handy. Basically, it would be yet another device to add to my arsenal of devices, but it sure can't replace all of them. They need to invent a device that can resize itself for that to happen.