I absolutely agree that no matter where the concept comes from it is key to realise you can't Get it ALL done ... at least not right now. The way I use GTD to help me with that is I use my horizons of focus list to prioritise. I look at my 50k and figure out my level of satisfaction with each item. And then I do that for the next level on down. And generally I pick 1-4 things I want to move up that satisfaction scale. There is no formula here. It could be just 1 thing because I know it's really difficult and it's going to take a lot of focus and besides it will likely positively impact everything else (e.g. working out regularly), or it could be 3 things because one I can concentrate on in the mornings, another in the evenings, and a third while at work. Then I make sure those are connecting to the runway with projects and/or next actions or routines.
Good point. It is sort of implicit in GTD that you will know your own 'bandwidth' limits. What do I have time and attention to take on at the moment? That's a difficult question. It's so easy to over commit.