MADRID, Sept 17 (Reuters) - The boards of Spanish lenders Bankia and Caixabank on Thursday approved the terms of a proposed merger that will create Spain’s biggest domestic bank with around 600 billion euros ($710 billion) in assets, sources familiar with the matter said. Caixabank and Bankia declined to comment. ($1 = 0.8450 euros) (Reporting by Jesús Aguado; editing by Sonya Dowsett)