You are right in that some of the best cb's have been small, Cannavaro being a case in point. The issue though with all the other quality cb's who have been on the small side, they have at least been fast or strong. Garcia is neither. 99% of the time it is footballers who are intelligent that we need in our team. The problem is the we have been caught out this season with that final 1%. It does not matter how much possession you have, how beautiful your football is or how great a team you are.If you are susceptible to one ball over the top, then the top strikers will punish you.
Garcia would be an asset with someone like Distin at one side of him and Kyle Walker on the other. The problem is that we are only as strong as our weakest link and when you weakness is the ball over the top into space, you can not have a cb who looks like he is running through treacle, regardless of how good he is on the ball.
If Pep thinks that Garcia is so good then keep him and build the defence around him. At the minute though his attributes are only exacerbating our major weakness.