I'm thinking he meant to say "size ISN'T that important to them"? This would make more sense in the context of the discussion.
In terms of his playing LB, it all depends on whether March can put on a good 30-35 LB by the time he's a senior without losing any speed. We don't know how extensive his strength training has been in the past. Maybe he has some upside in this area?
Mike Thomas (listed at 5'11", but probably closer to 5'9") was a pretty decent LB here, but his stature probably limited his ability to play at a higher weight (listed at 220 LB last year). Hopefully, March is a legit 6' (not that this is particularly tall for a LB).