Ensure it is the raw gba_bios.bin and not zipped maybe? I will check the exact size of my bios file tonight and let you know what it is. I tested Metroid Zero Mission the other night and it booted up fine.
I`m using the gba_bios.bin (16,384 bytes and MD5: A860E8C0B6D573D191E4EC7DB1B1E4F6) I think is the same bios of you, after I will try using Libretro gsps, thanks!