Abstract: A sizing composition including one or more primary lubricants, at least one secondary lubricant, and at least one coupling agent is provided. The primary lubricant is a fatty amide lubricant that is the condensation product of a saturated fatty acid having at least 16 carbon atoms and a polyamine. Additionally, the primary lubricant acts both as a lubricant and as a cationic surfactant in the size composition when it is neutralized with a weak acid to assist in dispersing the fibers in a gypsum slurry. The secondary lubricant is polyamide or a polyvinyl amine, and acts as a film forming agent in the fiberizing process to protect the glass fibers from interfilament abrasion. The sizing composition provides improved fiber feeding, fiber flow, and fiber dispersion in a gypsum matrix or slurry. The weight ratio by dry solids of primary to secondary lubricant in the size composition is at least 5:1.