The inherent brittleness of the refractory metal tungsten represents a major challenge for its application as divertor material in future nuclear fusion reactors. Grain refinement to the ultrafine-grained regime is a promising strategy to increase the fracture toughness of W, but it also promotes intercrystalline crack growth. Therefore, the strengthening of grain boundary cohesion in W is of great importance. In this work, grain boundary doping with B and Hf, two elements that were identified in previous work to increase bending strength and ductility, is applied to ultrafine-grained W. The fracture toughness is measured utilizing small-scale testing techniques. Fracture mechanical experiments on the microscale provide a plethora of challenges to correctly assess size-independent toughness values, which are presented and discussed within this work. It was found that the toughness of W can be under- and overestimated, depending on the sample dimensions and plastic zone size. When assessing the valid and size-independent fracture toughness measured for the differently doped W specimen, doping with the strengthening element B maintained the already remarkably high toughness of the undoped ultrafine-grained W of around 20 MPa√m. The samples doped with Hf even improved the fracture toughness to values of up to 27 MPa√m. Hence, the effects of GB doping on the fracture toughness of ultrafine-grained W are explored, while simultaneously the influence of sample dimensions on measured fracture toughness is discussed. These insights are expected to have a great impact on the development of superior materials for use in harsh environments, as well as the application of small-scale fracture mechanical experiments, as used, for example, in the assessment of control samples in nuclear technology.