Rossi's height matters little at the OHL level because he is pretty much already fully developed physically. He has tremendous lower body strength due to being a gym rat and lanky undeveloped OHLers have a hard time knocking him off the puck, even though he's very short.
When the players are men and are fully developed physically, he's going to have a harder time protecting and controlling the puck.
That's not to say he can't be successful because of his height at the NHL level, but he probably will have to adjust his game somewhat.