You mean what footprint is ideal for you, or do you really mean screen size?
Take the s3 vs the s4 for instance. The s3 is bigger but also has a smaller screen.
The lumia 800 has a 3.7" screen and is equal in size to the n9, which is a 3.9" screen.
People generally mean frontsize when they discuss screen size comfortability, so I was just wondering which you meant.
As for the poster saying that a 5" screen is useless on a mobile phone: no, it really isn't. Its quite a difference watching a movie on my lumia 800 as opposed to my s3, for instance. Even disregarding resolution. Same with webbrowsing. Any viewing activity, really, improves with a bigger screen.
Now, wether or not the added frontsize that brings is worth the better viewing experience, that's really a personal choice.
For me, Im happy with the size of a 920 (which fits a 4.5" screen but could easily hold a 5" screen with the same frontsize) and the size of the s3, which is why Im happily awaiting my s4.
As for overall batterylife, my s3 could do 6 hours of video and still have half the battery left, and generally held for 1.5 days where my 920 lasts about 0.7 days.
So some phones do well ime.