For 1 and 2, I would say it totally boils down to personal preference an distance/size ratio. For 3, again, distance to the screen matters a lot.
The 4th one I've seen the most heated discussions about. In my opinion, highest you can afford (both money-wise and computational power-wise) is the most useful resolution. Even if you can't distinguish the individual pixels (aka screen door effect) aliasing is still an issue.
1. At what size and resolution are flat screen monitors most useful?
2. At what size do curved screens start becoming useful?
3. What is the upper limit for useful screen sizes?
4. Is there an upper likit for useful resolutions?