This paper examines the influence of selected relationship marketing characteristics and customer attributes on outcomes of third-party Logistics relationships, and the extent to which provider size affects these relationships. An online survey was conducted, aimed at professionals working in German firms engaged in third-party logistics relationships. The data were analyzed using Partial Least Squares path modelling. The results indicate that provider size has only a limited impact on third-party relationship performance: Where an influence was observed, it appears that size has a negative effect on the relationship, particularly for customer referrals and retention. Implications are formulated