I'm no scout, but I listened to Deuce Windham and Ellias J. Williams podcast last night and they were talking about this very thing. They didn't seem to think size was an issue in coverage. But, they did say that while Queen is clearly the more polished LB in zone coverage and has great instincts in zone, Murray is better in playing face up in tight man to man coverage. They also noted that the Saints play a lot of man to man coverage with pressure on 3rd downs so it's possible that for things like covering a TE in man on third down or covering a back like McCaffery on swings out the the backfield, you really want Murray.
But they did say that either would be a very good pick in the first round. Sounds like it just depends on the kind of LB the Saints want and how they want to play defense against TEs and RBs. I'll add that I would expect that if Gronk can actually be Gronk again, they will use P.J. Willaims or CGJ to cover him rather than a LB when possible.