"Neymar will join Barcelona 100%, but has a problem!" . The advice doctors gave him before his first match :

"Neymar is 100% ready to integrate in Barcelona, ​​but needs to work slightly on his physical condition. He is a skinny player with a different conformation with the massive defenders he will have to fight. Has qualities that compensates strength, he has speed, throttle response, not to mention his dribbling skills. However he must put 2-3 pounds of muscle if he wants no problems. Anyway, adaptation of Brazilian players in Spain is fast, usually, and I think it will also be in Neymar's case" said Brazil's national team doctor Jose Luis Runco  on Sport.es. 

The same advice was given to him by  Barcelona's doctors on his medical visit. It is true that Neymar has no opponents in the defense side as if he had played in the Premier League, for example. Their strength is lower in Spain, because the championships differ.
 
At this point, with all the qualities that he has, Neymar is 21 years old, height 1.73 and weight 65 kg.