I have the smaller Pixel, one day in right now and so far I'd say my battery life is decently better than my Nexus 5X was which was a similar size battery (and much slower processor and dimmer screen).
I'm pretty pleased so far, but not blown away. I've had it off the charger since 7 AM this morning and I've listened to podcasts over bluetooth for around 3 hours and then used the speaker for a half hour. I've also been using it fairly heavily for Facebook and then checking my WoW Companion App throughout the day. Currently I'm at 33% after 14 hours since charge and 2 hours 11 minutes screen on time.
I have 2 co-workers with XLs and had been wondering if I made the wrong choice by going smaller, but the XL is too tall and heavy for me. I really wish that the regular Pixel was 5.2" and poly carbonate more in line with the 5X but eh.... it is a great and fast phone. I am impressed with the camera so far, key tip is use touch to focus if you really want the picture to be sharp.