About one third of the world population lives in earthen dwellings. Adobe connotes an ancient masonry whose bricks and mortar are made out of clay, silt sand and fibres, mixed using water and dried under the sun. The composition of adobe is not standardized yet, namely locally available soil and fibre materials are mixed together, often regardless strict rules on the nature and proportions of these constituents. As a result, adobe mixtures in the field are usually accompanied by a high level of heterogeneity at micro and macro scales. This requires proper assessment of the influence of heterogeneity on the mechanical performance of the finite products at macro-scale. A large portion of the thousands of new and historical adobe buildings in the world are indeed not engineered constructions, often built by house owners themselves with inherent safety issues. Moreover, this building technology is recently gaining significant relevance in light of its good sus-tainability features. This paper investigates the effect of heterogeneity in the mixture on the strength of adobe elements of different sizes. Size dependence is a well-known phenomenon for masonry elements. For indus-trially produced bricks, it is known that larger sizes are generally accompanied by comparatively lower strength values and several theories have been consolidated over the years. In this research, specimens of adobe mortar of different aspect ratios have been statically tested in compression. Nominal values of strength have been calculated and compared. Contrary to initial expectations, lower strength levels appeared to be as-sociated to smaller dimensions. First, this observation has been interpreted as a possible consequence of the effects of the heterogeneity level (sizes and distribution) in the mixture compared to mixture granulometry property. Mixtures which are not standardized and may compromise structural performance of comparatively smaller specimens. Next, this hypothesis has been numerically tested via a series of numerical simulations. A recently developed isotropic damage model called ‘Adobe delta damage model’ presented at EURO-C 2018, has been used to replicate the observed size effect. This model uses a damage delay framework to obtain mesh-size independent results for both static and dynamic loads in quasi brittle material simulations. Ex-perimental results, physical interpretation and numerical simulations are presented in this paper.