Aqueous peroxide-free size compositions for application to glass fibers contain, as essential components, a reaction product of a polymeric amine and an amine-reactable organosilane, a film-forming polymer and an emulsified polyolefin. The reaction product can be preformed or made in situ, and is preferably the product of a polyalkylene polyamine and an epoxy-functional alkoxysilane. An optional added component is a water-soluble, non-volatile carboxylic acid. Glass fibers treated with the size composition can be made into mats, especially continuous strand, needled mats, useful for reinforcing thermoplastic polymers such as polypropylene in laminates which then can be used in molding processes to make articles of good physical properties.