what is the max internal SD card size ?
- Someone got a 400GB working, but smaller internal SD's are usually better because they have faster random access/read times. A good Samsung/Sandisk is your best bet for the internal. Besides, unless you're running Linux to mount the internal card's ext2/3 partition, copying many GB of files over FTP is very, very slow.
can you put roms on both the internal SD card AND the external SD card ?
- yes. But some emu's requires specific, fixed paths. Symlinks could work if that word doesn't sound frightening
I've heard that the HDMI doesn't work, but does the AV out to composite work?
- no AV, probably ever. HDMI is supposedly being worked on.
Can you plug in an external USB controller ?
- yes. Some work. TonyJih posted a video of this
Can you plug in an external USB stick to one of the USB-C ports and run roms from it ?
- seems like it should be possible but haven't seen any confirmed reports