By Mary Milliken LOS ANGELES (Reuters) - In preparing for her first film role as a petite, hard-laboring slave in "12 Years a Slave," actress Lupita Nyong'o sized herself up against 500 pounds of cotton, which her character Patsey picked every day. The pile "was taller and wider and thicker than me," says Nyong'o. "And I was presented with this woman's loftiness." Conveying that loftiness proved to be a key to Nyong'o's performance, one that has generated critical acclaim for the Hollywood newcomer and predictions she will earn a best supporting actress Oscar nomination. "12 Years a Slave" is based on the 1853 memoir of Solomon Northup, a free black man tricked and sold into slavery who cannot fathom the hell he has landed in on Louisiana plantations.