How big can a smartphone get before it gets impractical? It depends on who you ask. The answer is more complex than just one number, like 5". It depends on the form factor. With a very wide screen you might have trouble using it with one hand, or even holding it with one hand. If the screen is very high, you might have the same problem.
There's of course a carrying issue with larger phones. A 5" phone might not fit well into the front pocket of your trousers and the back pocket is just too dangerous a place for a smartphone. I usually keep my iPhone in my breast pocket, where it's out of the way of keys, coins and the rest. BTW a silicon phone protection is essential for breast pockets, because it stops the phone gliding out of the pocket. Weight is an important issue too.
Now some people just insist on having big smartphones, and there's a solution for them too. At least if their big one is an HTC Butterfly (5"). Here's the solution: an HTC mini:
The HTC Mini has a number pad and small screen and can be used as a handset companion to the Butterfly (the non-US version of HTC’s Droid DNA) as well as a remote control when the Butterfly is connected to a larger display. The Mini can make calls, display messages and calendar events, and control the shutter on the Butterfly when connected.
It's inevitable that screen sizes will become more diversified. I think size will become fashion. Sometimes small will be all the rage, sometimes big will. I also think pads and phones are so different devices that there will "always" be room for both. Both have their different strengths.