This means computers are getting smaller and more powerful. For instance, according to this website, your iPhone 5S is 1,000 times more power that 1975's "supercomputer" the Cray-1.
Think about that. You hold in your hand a computer more powerful than 1,000 former "supercomputers" that probably cost millions of dollars, each.
Ain't technology grand. What used to take up a room is outpaced many times over by something that fits in your pocket.
However, as computers shrink, I think there's a practical limit on how small they can become. Let's call it the "biology barrier."
At some point, you have to have input and output on a computer. Unless you're going to wire the computer directly to the brain (which is a little scary, what if you get an email with a virus attached?) you're going to have to deal with fat human fingers and human eyes/ears. Maybe they can perfect the voice interface (Apple's Siri is amazing sometimes, sometimes as dumb as a Cray-1) but voice interface is slow (slower than I type on a full-sized keyboard) and accords no privacy. Can you imagine standing in line at Starbucks sexting your girlfriend with a voice interface?
So, I'm thinking the device can be small (but not too small, don't want to easily lose it) and have a holographic screen that projects out of it and input is by "touching" that screen. If it were a watch-sized device, worn on the wrist, that might work but then you only have one hand for input.
And I don't want to give up my full-sized mechanical keyboard unless your holographic keyboard had "feel" to it.
I don't think, even as they get more powerful, computers for direct human use will not get much smaller than what we have right now. Below that, input is too hard and output is too small.
But you never know. My grandchildren might be using something that's a button on their shirt with holographic input and output and laughing at me with my "huge" smartphone.