Don't know anything specific about Warriors from grass, but it'll depend on how big the people are, how wet the grass is, how long it is, what the slope is, what the temp is, how much fuel and other stuff you have on board, what condition the engine is, what the wind is etc. etc. No simple answer and I doubt there are many 4-seaters you could operate with absolute impunity out of a 650m grass strip. Maybe a 235hp Maule? Maybe a Cessna 180/185/206? Dunno.
Not a helpful comment, I suspect, but a reflection of reality.
QDM