Understanding the interplay between cell fate specification and morphogenetic changes remains a challenge in developmental biology. Gastruloids, stem cell models of postimplantation mammalian development, provide a platform to address this question. Here, using quantitative live imaging and transcriptomic profiling, we show that physical parameters, particularly system size, affect morphogenetic timing and outcomes. Larger gastruloids exhibit delayed symmetry breaking, increased multipolarity, and prolonged axial elongation, with morphogenesis driven by size. Despite these variations, transcriptional programs and cell fate composition remain stable across a broad size range, illustrating the scaling of gene expression domains. In particular, extreme sizes show distinct transcriptional modules and shifts in gene expression patterns. Size perturbation experiments rescued the morphogenetic and pattern phenotypes observed in extreme sizes, demonstrating the adaptability of gastruloids to their effective system size. These findings position gastruloids as versatile models for dissecting spatiotemporal coordination in mammalian development and reveal how physical constraints can decouple gene expression programs from morphogenetic progression.