I think this is basically going to be the size of the Wii U tablet basically, maybe shave a bit off the bezels and make it thinner/sleeker, but even at that size with a depth of 0.75 inches lets (this about the same as a Vita) ... the internal volume is huge, it would be bigger than an Apple iPad Pro in volume because it's 3x the thickness almost.
It needs to have some thickness to it anyway because gaming on a device with physical controls as thin as an iPad (0.27 inches) won't work well at all.
That should be enough to get a monster battery and still have enough open room for cooling.
The idea will be not that this is a pocket device, but that it's a portable table-top console that encourages people to gather around and play and if its for one player then the hook is that its better than a tablet at dedicated gaming.
But I see this at airports like a million times over, families with young kids gathered around a tablet playing. I think Nintendo has studied carefully how kids use tablets too. It will probably have a built-in kick stand like the Microsoft Surface line of tablets too.
