[kamocat]

If you're asking about an upper limit, I would say around 10 kilo-ohms, but you could probably get away with a 50 kilo-ohm pot. Each input has a 1 mega-ohm resistor inline.
If you're asking about a lower limit, that is about 20 ohms if it's the only thing on the analog breakout. The limitation is that the 5v supply on the analog breakout has a 300mA limit.

So, anywhere between 300 ohms and 3,000 ohms is great.