Bruschi was 6'1 247, Kendrick is 5'11 237. Bruschi is the smallest LB BB has used, so smaller than the smallest certainly would be a concern.
Other LBs in other systems are not a good comparison. Kendrick could fit well on other teams, but it seems like he would be a bad fit given what BB asks of his LBs, Of course everything evolves and there could be a role, especially as the league continues to get more pass heavy, but not a big one.
If we are in nickel 75% of the time in a 60 snap game that gives you 135 total LB snaps. The issue is more that this isn't enough snaps for the 3 we have and we have to find other ways to use them (such as HT at DE, or playing 3-3 nickel) than that we need more LBs.