During multi-cellular development, highly reproducible gene expression patterns determine cellular fates precisely in time and space. These processes are crucial during the earliest stages when the body plan and the future asymmetric body axes emerge at gastrulation. In some species, such as flies and worms, these early processes achieve near-single-cell spatial precision, even for macroscopic patterns. However, we know little about such accuracy in mammalian development, where quantitative approaches are limited. Using an in vitro model for mammalian development, i.e., gastruloids, we demonstrate that gene expression patterns are reproducible to within 20% in protein concentration variability, which translates to a positional error close to a single cell diameter at the tissue scale. In addition, 2-3 fold system size changes lead to scaled gene expression patterns again on the order of an individual cell diameter. Our results reveal developmental precision, reproducibility, and size scaling for mammalian systems. All three properties spontaneously arise in self-organizing cell aggregates and could thus be fundamental features of multicellularity.