These days, the line separating phone and tablet is getting very blurry. At what point do you call a device a tablet? Recent mobile devices having screens larger than 5" have brought on this new term called a "phablet." It's not quite a phone, but it's also not quite a tablet.
I don't think a 5" device would fit too comfortably in pant pockets. But, it may be fine in a purse or a white coat pocket. Putting one on your belt could be awkward if you're a fairly smaller person. The recently announced Samsung Galaxy S 4 is a 5" smartphone that some may call a phablet. It's similar in size to the Galaxy Note 2 that supports an active S-pen stylus.
I still remember the days of the OQO model 02 which was a 5" PC that ran Windows XP, Vista, or 7. It was a full PC. It was heavy, bulky, and had short battery life, but it was the perfect device for people who needed a full PC in their pocket.
Like many people, I find myself carrying multiple devices. When I travel for business, I carry an ultrabook so that I can do some quick typing and maintain a high level of productivity. I also carry a small tablet for quick reading, scribbling notes, and email/web browsing on-the-go. And then, I have my phone in my pocket. Perhaps that's all going to change soon. Maybe we'll find ourselves only carrying a single device that's capable of everything. The mobile device will dock into a workstation that will allow us to be very productive. We'll be depending more on cloud computing solutions so that the device and the operating system will become less relevant (except for the need for greater security within the OS).