Any way to make a Nexus 6 sized prototype? Do I need to use Nexus 5 or 9 body & scale assets in some manner?
No, you can create your own device sizes/previews: https://framerjs.com/docs There is a section "Device"
You can check out the new Custom Device Snippet for an example of this as well, Matt Marriotti (Snippets → Custom Device). :-)