You can use either mini SD or micro SD (the latter with an adapter micro-mini). You can't use regular-sized SD.

You can use any capacity you can get your hands on. I'm not sure what the largest micro/mini SD is, but I think I saw somewhere that you can buy 8GB micro and 6GB mini (micro being a more "trendy" format). Obviously these are implicitly of SDHC type. If there are larger cards available in the future you can use them all.

What you cannot do (and why I'm not buying an N810) is putting in a regular-sized SD card.

And the N810 has a single slot, mini-SD sized.