That or working and trying to look at plans on your phone so you're not destroying them shoving them in and out your vest, trying to carry a bunch of equipment around. Here are some of the easier to read examples of stuff I would be using my phone to view. The smaller the phone, the more zooming in to see. I couldn't quickly find any of the more difficult to read plans I sometimes am trying to read.
I'm probably in the minority but for me it's a mini computer which is used all day long for viewing documents, email, phone calls, texting the project manager, using as a WiFi hotspot, calculator, unit converter, music player etc.
So the bigger the better in my situation. I went for basically the largest available (6.8"). Typically the bigger the phone the better the battery life when being used heavily.
For people that just need a smartphone to cover the basics there are lots of phones 6.1" or smaller available. though.