When the iPad first came out, I thought that having a larger size would be perfect as a replacement for carrying a laptop around. But now after living with iPads for the past couple of years, I feel different about it.
Going bigger simply forces the use of a device into what effectively is a laptop, much like Microsoft's Surface. Sure, you can hold a larger screened device, but to do any work on it, or even interact in a basic way, you really would need to set it down. Holding a device one-handed while interacting on-screen, is not an efficient way to do anything, other than maybe browsing online or reading emails.
Using a keyboard in the same plane as your arms/hands in front of you (while supported by a desk or some other flat surface) is still the best (read as easiest physically on your body) way to interact with a device. If Apple can create a MacBook Air with a thinner form factor that is essentially an iPad screen with a keyboard and trackpad (maybe supporting an additional battery for longer use), I would consider that an ideal work travel computer. I don't need a bigger touch screen, which will show that much more fingerprints.