Heiskanen is a late developer physically plus with the July birth date he isn't among the oldest draft eligible players anyway. I think his size has made people overlook the talent previously, but now things are obviously changing when he is catching up on that. He is now pushing 6 feet and I'd predict that he will be 6'0, 190 when it's time for combine measurements.
I don't think his size will be a problem. He doesn't even shy away from contact, likes to use the body when battling along the boards etc. Of course he is never going to be a physical player on international level so he has the develop the other skills a little more than some bigger guys. However, playing always against older competition as a undersized player forces you to play smart and think quick. That's what Heiskanen does.
The scouting report on the OP seems quite accurate. One word the describe Heikanen's game could be proactive. He seems to want to make things happen on the ice, both on offense and defense, instead of staying back and seeing what happens. He tries to stop the opposition even before they enter the offensive zone and goes for takeaways on defensive zone. Of course this style means some plays can be majorly wrong too. This is the area that needs the most work, experience and maturity.
Like said, Heiskanen definitely likes to join the rush as a third or second man and his slick skating helps a lot on that. I don't see his shot as the biggest strength, but his passing in the offensive zone is very fast. He seems to have the next move always figured out before he gets the puck.
It's not really surprising that he is climbing in the ratings. So far he has played in two U20 tournaments for Finland this season and the coaches have commented positively on his play in most games. For IFK he has suited up for one minor tournament and two CHL games, again getting good reviews in all of those games. He is a player who has fast feet and head and good skills. I think a lot of NHL teams like that these days.