Sediment permeability is a key parameter controlling overpressure development and fluid egress at subduction zones. Existing data compilations for argillaceous sediments illustrate that scale effects on permeability are generally minor, although this is a topic of active debate. Here, we report laboratory permeability measurements from sediments incoming to the Costa Rican subduction zone, at porosities from 26% to 78%. We then apply a method for directly estimating average permeability at the scale of sediment layer thickness, constrained by published estimates of excess pore pressure. Permeability values from both methods are in excellent agreement, and exhibit a trend consistent with permeability-porosity relationships used in numerical models at scales of several to 10's of km. Our results indicate no scale dependence of permeability, and strengthen previous interpretations drawn from more limited data sets. The permeability-porosity relationship we derive is comparable to those inferred at the Nankai and Barbados subduction zones, indicating that the high porosity, and hence high permeability, of incoming sediments at Costa Rica is the most likely explanation for comparatively rapid dewatering there.