Speed
Smarts
Strenght
Skill
All more important then size. Sure size gives you better reach which is good and probably a higher ceiling speed/strength wise, but probably gives you a lower floor too. If you look at MSL, he was a super strong player even tho he was small... Gaudreau is probably one of the weaker player in the league, but all the other attributes are elite. In conclusion, no size is not a game breaker for a prospect/player
Yes and no. I will not deny your basic premise, but... Taken as a generic whole, the overall group of players who even have a shot of making the NHL is a tiny, tiny, tiny percentage of the generic group of 'hockey players' around the globe. At those rarefied heights, the differences between player 'x' & player 'y' aren't 'that' great (compared with the difference between them & say a lower tier AHL player who in turn is incredibly more skilled than the average beer leaguer). All things being equal, & at that level they pretty much are, a team will generally go with a 'bigger' player than a 'smaller' one. The outliers you mentioned (MSL & Gaudreau) are just that, outliers due to their far superior skill in other important areas.
I haven't checked myself, but I'd bet that if you looked at your favorite team's roster from say 6-7 years ago and looked for the smallest (non superstar) players on there (ones who got regular playing time & was a genuine 'NHL player', not just someone who stopped by for a cup of coffee - someone in the middle of the depth charts) you'd be asking yourself "Oh yeah him, I wonder what happened to him...". Not that there was anything necessarily wrong with them, but as a 'generic NHL player' they're completely interchangeable & chances are good that they were thanked for their time, shown the door, & replaced by someone younger (& most of the time, bigger). On the same roster you'll probably find players who stuck around even though they too are 'average', I'd be willing to bet that they were on the upper side of the 'average NHL players size' chart.
Size or lack thereof is more important than you are taking it, just because all else is pretty much equal. That said, it wouldn't surprise me if the average size in the NHL actually goes 'down' a bit moving into the future. However that's probably more a factor on the big (& good) crackdown on headhunting & other highly damaging hits than teams wanting smaller players. The smaller, 'very' skilled players now can survive the in game long enough to show that they're better than the bigger & slightly more 'average' player. But that's a discussion for another thread...