CHICAGO June 26 (Reuters) - The family of Mexican billionaire Carlos Slim said on Thursday that Circuit City Stores Inc. (NYSE:CC - News) had rejected a takeover offer, sending shares of the U.S. electronics retailer up sharply amid hopes of a higher price. The $8 per share offer from the Slim family, which owns computer retailer CompUSA and a large stake in upscale retail chain Saks Inc. (NYSE:SKS - News), comes as Circuit City struggles with slumping sales and losses in its credit card operations. The Slim family, whose holdings in Telefonos de Mexico (Telmex) and several other Latin American...