The influence of pollinators on the evolution of flower morphology has been extensively explored. Yet, the effect of other ecological factors, such as genetic drift, environmental filtering, and allometric constraints, gained less attention. In this study, we addressed the importance of those drivers in a predominantly selfing species. 400 worldwide Arabidopsis thaliana accessions were gathered and grown in semi-controlled climatic settings to explore the association between flower organ size, genotypes, and habitats. In our dataset, petal area was the most variable trait. Petal size was phenotypically and genetically correlated with other flowering structures, but no genetic allometry constraints were found to affect petal size evolution. The negative correlation of petal size with fitness and the traces of selective constraints in petal associated genes suggest that petal size is currently under selection in this species. We found paucity of genotypes harbouring large petals at low suitability regions, which points to the presence of environmental filtering. The novelty of this project relies on the pluralistic integration of factors studied and highlights the role of the climate on flower size evolution. Our results suggest that resource allocation is an important driver of flower size evolution in self-fertilising species but that its effect is largely determined by local environmental pressures.