At what point is a PC too small?

We live in a world of mobile technology. We see PDAs, BlackBerrys, iPhones, Smartphones, iPods, and all sorts of other mobile devices. Computers are also getting smaller and we see young people using UMPCs, netbooks, micro PCs, and other miniature computers. But at what point is the PC so small that you fail to be productive? At what point does the small size hinder your ability to do what you need to do? How do you balance size, weight, and functionality? After all, you can probably find a PC in a very small size, and then if you look some more, you can probably find another one that's 25% larger. So how should a physician, pharmacist, or nurse decide on the size of that mobile PC when working in the clinical setting? At what point is an ultra-mobile PC too small?