The durability of wood in ground contact is affected by its material resistance on the one hand, and the exposure situation in the ground on the other hand. The latter is considered to be one of the most severe not at least due to permanent wetting and direct contact to a well-established microbial flora. In addition to physical, chemical, biological, and ecological soil parameters, the design of a wooden commodity which is in contact with the ground can have an effect on its durability. This study examined the effect of size of specimens used for in-ground durability tests. Standard EN 252 specimens, smaller mini-stake specimens, and larger double-size specimens were made from Scots pine sapwood and heartwood (Pinus sylvestris L.), Norway spruce (Picea abies Karst.), beech (Fagus sylvatica L.), and English oak (Quercus robur L.) and exposed in ground in a test field in Hannover-Herrenhausen, Germany. In addition, standard size specimens were exposed on the ground. Decay rates and corresponding durability classes according to European standards were determined. Decay proceeded slightly faster with decreasing specimen size, but for the majority of the tested materials no significant effect became apparent. However, the most durable material tested was English oak, for which durability was clearly affected by the specimen size. It was classified ‚durable’ (durability class DC 2) using double size stakes, ‚moderately durable’ (DC 3) using standard specimens, and ‚less durable’ (DC 4) using mini-stake specimens. Specimens exposed on-ground decayed significantly less rapidly compared to specimens buried in the ground to half of their length. The findings from this study recommend to use also test specimens, which are bigger dimensioned than standard specimens and thus closer in dimension to real size commodities. Otherwise, one might accept to underestimate the durability of particular wood-based materials.