A friend of mine told me that a 32-GB SD card would be sufficient for Retropie. He was wrong. I upgraded to 128-GB SD card and because I didn't format the previous SD card FAT32 I couldn't just create an image and copy that to the new SD card. However copying the Retropie image is pretty quick it's just copying the ROMs back over which takes long. I started a file copy of the ROMs overnight via FTP and then for some reason the 5-GHz WiFi crapped out in the middle of the night and I had to reboot the router the next morning. So I restarted the file copy and Retropie was back up and running by the time I came home from work.
Right now I'm emulating Atari 2600, 7800 & Lynx, Game Gear, Game Boy, Game Boy Advance, Game Boy Color, MAME (two different ROM collections), Master System, Genesis, N64, NES, PC Engine, Playstation, SG-1000, SNES and Vectrex. I think I'm up to like 10,000 games or some shit. This thing is awesome.
I haven't been out of the house in 3 days and haven't showered or brushed my teeth either. I've just been playing video games with occassional breaks to get on AtariAge.