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I'm curious, what's the biggest capacity memory card (either available or theoretical)? Are any games incompatible with non-official sizes?

Pardon me for being stubborn about this. The ability to save progress to the cartridge during gameplay is essential.

I don't know specs off the top of my head, but I'm pretty sure Sega only released one size Backup cart. At least in the US. I don't know if the Action Replay cart has more memory, but you can't save in-game with those. This is not to be confused with RAM carts, which add extra RAM for the console. Those came in 1 Meg and 4 Meg sizes, and cannot be used for saves.

OK. Most likely I'll look for backup cart that has region free functionality.
I can live without RAM or cheats.

The only carts I know of that can boot games from other regions are the GameShark (do not use this - it will fuck up your cart slot because the PCB is too thick) and the Action Replay. The Action Replay can also act as a RAM cart (both 1 and 4 Meg), cheat device and store save game data. The only downside is that you cannot save to the AR in-game (you can only transfer to/from the system memory). But honestly, you can get the Saturn AR brand new for $30-35 shipped. Every Saturn owner should pick one up. It's like a Swiss Army Knife for the Saturn.

Also, I noticed a seller on eBay selling Sega brand Saturn Backup RAM carts for $12 shipped. They're JPN versions, but can be used on any NTSC console.

After a GameShark fucked up my Saturn's cart slot, I'm very apprehensive about trying other 3rd party carts. The Action Replay never gave me a problem, but I'll stick to the genuine Sega Backup RAM cart. That Saturn cart slot has such tiny pins, they seem to be very easy to distort.

A JPN memory card goes well with my white Saturn.
Half the games that need RAM are just 2D fighters, no loss there.