Re: Mifi+Tablet+(VOIP(Sipgate/Skype/Other))=Phablet
It's a weighing up of many factors, including what they use the phone for, the clothing they wear, the size of the hands etc etc. Jo Public has gone through a few generations of phone since the first 'Phablets' arrived on the market, and so he has more than enough first-hand information (his previous phones, handling his friend's phones to get an idea of the size etc) to make an informed choice. All design is compromise.
There are two longstanding products I like to equate to smartphones. Playing cards are designed to fit in the hand easily, like an iPhone 4. Postcards are a good size for displaying a more detailed image, like a 5"+ smartphone.
We used to consume our news on a 24" 'screen' that could be stowed in a jacket pocket. As a bonus it could also be used to wrap our chips, swat flies or scrunched up and placed in wet shoes. It was called a newspaper.