I don't know what you mean about the keyboard, since I would literally use the same keyboard size if I were to have an external one (I'm on a Macbook Air, and would use the bluetooth mac keyboard). Never had a problem and I'm using my laptop quite extensively for everything.
The head down thing though is valid. It can be mitigated by having a stand or a monitor. I mostly deal with it by just taking a small break every now and then, although that's more related to sitting down than my head position.
I don't see this mentioned often but the ergonomics of a laptop are rather poor, you'll end up with RSI much quicker that with a desktop. And this can become quite debilitating.
I don't think this is an issue for a couple of hours usage a day, but I would not want to spend all my computer time on a cramped keyboard with my head down.