Apparently 8x8, for most characters traditional Chinese characters: https://imgur.com/DBRSqIn

Probably lower for simplified Chinese and katakana/hiragana Japanese characters.

I'd say that at 2x2, "Two Slice" is definitely not readable.

8x8 for Chinese is difficult, but I think it’s a bit less guesswork than 2px high for English. I wonder what simplified Chinese would look like 6px tall.