Raspberry Pi's don't use PXE boot, they have their own proprietary system. The kernel is fetched by TFTP, along with a config text file which species an NFS location to mount as root. Boot then continues from there.
They estimate ~155MB just to get PHP running in Workers. It will be more to load WordPress. And there is extra work to get it running with Cloudflare D1. Plus everything else that likely won’t work by default there (e.g. image uploads).