Betsy Morgan and Lewis Cleale age gracefully in Signature’s “Giant” Let’s get one thing out of the way early – Signature Theatre’s Giant is worth the four (yes, four) hour investment. The new musical from Michael John LaChiusa was gaining notoriety for its epic length even before it opened (Signature itself took the tactic of joking about it in its marketing materials, calling it a “Texas-sized musical”). But the work, based on the Edna…