13" (what they call version 2 or v2) but not a ton of hours on it yet. When concerned with battery, my main use case is low brightness with text editing, reading, and/or web browsing. I think I can expect at least 6-8 hours of this activity, and maybe 4-5 hours of video watching on medium brightness. But not enough experience to confidently state these numbers.
A big question is if you'll want to stick with PureOS or install a different distro. So far I've given PureOS a shot, for fun and to see how I like it, and I'm used to Debian-based so it's okay although feels a bit dumbed down. What I don't know, is whether it's easy and likely-successful to install other distros -- it may be, I just haven't done research on that yet.