Fiber-reinforced polymer composites are widely used in applications where they may be subjected to extreme strain rates. Typical composites comprise stiff fibers coupled by a narrow interphase region to a surrounding polymer resin. The performance of composites is partly dependent upon their failure mechanisms, which can include fracture within either material, or debonding at the interphase between the materials. The nature of the interphase, particularly the strength of the interfacial bond, strongly influences the failure mechanisms and thus the overall performance. Here, we describe our computational and experimental studies of glass fibers embedded in a novel thermoset matrix, polydicyclopentadiene (pDCPD). To make the fibers compatible with this material, we have undertaken the design of new fiber coatings (i.e., sizing packages) intended for the non-polar, hydrocarbon chemistry of pDCPD. Our approach to this design process includes experimental measurements alongside a multi-scale computational methodology. At the nanoscale, we use reactive atomistic molecular dynamics (MD) simulations to determine the effects of the surface density and chemistry of fiber/polymer coupling agents on the interfacial shear strength (IFSS) and mechanisms of failure. The MD simulations help parameterize higher-scale finite element (FE) models of experimental single-fiber fragmentation tests (SFFT), improving our understanding of the design space with information from across multiple length scales.