You have to define the limit for the binddn you use in your syncrepl definition ( 'cn=user,ou=login,cn=system' here), or change this binddn to be the one for which you have an unlimited sizeLimit.

Thanks for the advice Emmanuel.

I finally put these lines on the global limits : /sizelimit size.soft=unlimited size.hard=1000// //timelimit time.soft=unlimited time.hard=5

/Tell me if i'm wrong but the hard limit is the absolute time /size the server will spend processing the request.