I wouldn't say Mateychuk has nearly as much of a size issue. He just needs to work on D technique and learn to pick his spots vs pros. That last part might have to involve playing him through some rough patches in the NHL next year (or maybe AHL the following season). Jr really is too easy for him in some ways.
Yeah, there just isn't much CBJ can do about that CHL rule, it is what it is. On a positive note however people say he seems to have mature pro mentality towards the game and his personal growth as a player which makes me hopeful that he will concentrate on getting better everyday throughout his junior career instead of starting to "coast" in his final year(s) because he knows he can afford to do it at that level.
Making the WJC team this winter would be beneficial for him in the sense that he would presumably face more near-NHL-level forwards. I got the impression he impressed Team Canada brass at the summer camp so he should at least have a good shot at it as long as he avoids injuries and major scoring slumps.
I know watching videos may be a little artificial method to improve D technique and such matters but I'm sure that going through the tapes of "Hockey Defending 101 With Nick Blankenburg" would also be a good way to prepare for NHL competition.
Interesting to note that Mateychuk is playing RD for Moose Jaw so far this season
He played on both sides last year as well. I think they still have more LHDs than RHDs on their roster which has probably caused them to move one or more LHD on the right. Mateychuk is pretty interchangeable rover-type anyways thanks to his puck skills and mobility so all things considered that move possibly makes most sense for the team.