sudo dd if=/path/to/batocera.img of=/dev/sdX bs=4M status=progress conv=fsync

This is the critical bottleneck. Batocera itself takes up about of space (depending on the version), leaving you with roughly 24GB to 26GB for games.

Even a 20-year-old PC can often handle systems like the NES, SNES, Genesis, and PlayStation 1. Finding the Right Download Link