The domestication of grain crops is among the most important phenomena to facilitate humanity's cultural development, and seed size increases are taken as one of the earliest domestication traits. Much remains unknown about the ecological drivers and cultural mechanisms surrounding this trait, but morphometric analyses have been crucial to investigate the topic for decades. Measurements on ancient cereal grains show that they evolved to produce larger seeds in their region of origin prior to dispersing beyond their progenitor range. This paper takes a transcontinental (Europe and Asia), long-term approach to comparative morphometric data. Unpublished measurements from over 10 sites of barley, free-threshing wheat, broomcorn millet, and foxtail millet from Central Asia and China have been collected for this study. We have contrasted these with published data from Europe, southwest and Central, East and South Asia. We investigate whether these cereals evolved in parallel or divergent ways across different lineages after they dispersed from their centres of origin; we trace seed size changes from initial cultivation through their spread and eventual adaptation to novel environments. This comparative analysis allows us to discuss rates of evolution and highlight evolutionary trends within some of the most important cereal crops across the Eurasian continent.This article is part of the theme issue 'Unravelling domestication: multi-disciplinary perspectives on human and non-human relationships in the past, present and future'.