Big enough that I lose interest or notice the padding.

A lot of it boils down to execution. The more urban areas of a Sleeping Dogs or the TW3 map with the Bloody Baron (not the viking map) feel geuinely massive enough though both are on the smaller end. Whereas something like GTA5’s San Andreas actively pissed me off because so much of the game was just driving to and from set pieces on the interstate.

That said: I actively don’t care about completion unless I really love the game. So if something was 40000km^2… I might never leave the two square kilomters the actual game takes place in and not care about the rest.