I think, it has to be 50+ blocks with mushroom grass. but the mushrooms that grow on it also count. I think I read that. so, theoretically, less than 50 could work, let's say, 40, and the mushrooms and mushroom trees that grow on it should make it count as valid, I think. But 50 should work anyway.
No need to make it as big as the 1.4 underground ones - but it can has its own benefits to make it larger, since then some unique monsters that spawn only on surface mushroom biomes might spawn more likely. (spore zombies)