The point to mention his size along his skating ability is that being a good skater a 6'5 is good enough to make it to the NHL, but at 5'10 not really.
About your comparisons, all those are more multi-dimensional, Fonstad is going to be a one-dimensional playmaker at the NHL level. Gallagher shot is elite and his feistiness is many steps higher than the average NHLer which makes him an effective and special player. Domi was despite his size a multidimensional true first round talent, feisty and skilled. Hudon is the fairest comparison, has similar skating to Fonstad imo, and was used at the point on PP in the Q not only because he was the go-to setup guy but because his point shot was a threat that was used as well, and he managed to make team Canada at 18 years old because of his 2-way game where he was planned to play a defensive role (got injured). Hudon had all this going for him but to me, he is still a pretty fringe NHLer, he seems like he will be able to stick around as 3rd liner, because of his 2-way intelligence he doesn't cost his team defensively despite his smaller frame. I don't think Fonstad could survive in a similar role, he might cap at where Scherbak is right now, the skills are evident but can he truly find a role where he can positively contribute to the team. Fonstad could be a home run pick if his playmaking ability continues to blossom along with improvement with his skating but like I said I have my doubts. All this being said I am fine with this pick considering he was taken in the 5th round.