A fiber size composition contains a modified polyolefin, a hydrophilic coupling agent, a boron-containing, fluorine-containing compound, a blend of at least two fatty acids and a compound selected from phosphorus(V) compounds and sulfur(VI) compounds, the fiber size composition being substantially free of conventional lower oxidation state antioxidants and optical brighteners. Composite materials produced from reinforcing fiber materials sized with a fiber size composition according to the invention exhibit improved properties such as, for example, increased strength and/or improved color stability.