To be fair, we already have a number of ex players working at the club - e.g. Ferreira, Cudicini, Hilario (is Eddie Newton still at the club?). We are just bringing back the bigger names for bigger roles, I guess.
Bulking up a bit would help but think the most important thing is, whether he can be aggressive and show tenacity when needed. As @OneMoSalah mentioned above, there are a lot of similar build players (from the Bundesliga or elsewhere) and they can handle the physicality of the Premier League. Also, think back to Oscar. Had (still has) a small looking frame but showed he can handle the physical demands in England.
TBF, it wouldn't have been that many had Lopetegui not done what he did and even then, Hierro was only the caretaker and Enrique had to quit due to personal reasons. It's not like they were happily sacking people...