Both the 5.2" and 5.5" should last you the whole day with light usage. Used the 5.2" previously for a week, and the battery life is good.
The phone is underrated because people prefer specs rather than overall performance. To me as a user, it is actually doing pretty well. I do feel the upgrade coming from Xperia Z3 Compact, with better screen, better camera, better overall performance and better everyday battery life. The CPU performance isn't that bad compared to Snapdragon 650 and 652 but the GPU performance is noticeably lower, but I still manage to play all the games I'm playing on my phone smoothly.