Maximum weight 8kg including the carrier, J cabin is fine.
It can be expensive, they charge 50€ for the LHR-MAD and 150€ for the MAD-USA, each way.
Several breeds are not allowed (seems to be a health issue for the animal; I think they are all dogs that have been bred to have a short snout): Affenpinscher, Boston Terrier, Bóxer (todas las razas), BrusselsGriffon, Bull Mastiff, Bulldog (todas las razas), Cane corso, Carlino(todas las razas), Cavalier Spaniel, Chihuahua, Chin japonés, Chow-Chow, Dogo de Burdeos, English Toy Spaniel, Grifón de Bruselas, Japanese Spaniel, King Charles Spaniel, Lhasa-apso, Maltes, Mastiff, Mastín (todas las razas), Pekines, Pinscherminiatura, Pitbull, Presa canario, Pug, San Bernardo, SharPei, Shich-tzu, Spanielinglés enano, Spanieltibetano, Yorkshire terrier.