A sizing composition that permits in-line chopping and drying of reinforcement fibers for reinforcing thermoset resins is provided. The size composition includes at least one coupling agent and one ormore blocked polyurethane film forming agents. The blocking agent preferably de-blocks at a temperature that permits simultaneous or nearly simultaneous de-blocking and curing of the polyurethane film former. The sized fiber strands may be chopped to form chopped strand segments and dried in a fluidized bed oven, such as a Cratec(R) drying oven, in-line. The chopped fiber strands may then be usedin a bulk molding compound and molded into a reinforced composite article. Chopping the glass fibers in-line lowers the manufacturing costs for products produced from the sized fiber bundles. Further, because the reinforcement fibers can be chopped and dried at a much faster rate with the inventive size composition compared to conventional off-line chopping processes, productivity is increased.