Coming from the Nexus 5 I have found I have adapted to the slightly smaller screen very well. Honestly think the screen is about the right size for me, love the proportions of the Z3c.
I concur, the phone is virtually all screen and thankfully it doesn't have the large bezels like its older sibling, which helps a great deal.
It's almost as fast as the N5, but I'm not missing the dire battery life on the N5 that's for sure!