The present invention relates to a sizing composition for use on glass fibers which are used to reinforce plastic resins used in the formation of fiber-reinforced plastics (FRPs). The sizing composition comprises a copolymer formed from at least one functional monomer that is selected from the group consisting of hydroxy, phenyl and alkoxylated functional groups. The glass fibers sized with the sizing composition of the invention have excellent computability with resins used in forming FRPs, as seen by a low level of white glass.