The binding of a T-cell antigen receptor (TCR) to peptide antigen presented by major histocompatibility antigens (pMHC) on antigen-presenting cells (APCs) is a central event in adaptive immune responses^,. The mechanism by which TCR–pMHC ligation initiates signalling, a process termed TCR triggering, remains controversial^,,. It has been proposed^,, that TCR triggering is promoted by segregation at the T cell–APC interface of cell-surface molecules with small ectodomains (such as TCR–pMHC and accessory receptors) from molecules with large ectodomains (such as the receptor protein tyrosine phosphatases CD45 and CD148). Here we show that increasing the dimensions of the TCR–pMHC interaction by elongating the pMHC ectodomain greatly reduces TCR triggering without affecting TCR–pMHC ligation. A similar dependence on receptor–ligand complex dimensions was observed with artificial TCR–ligand systems that span the same dimensions as the TCR–pMHC complex. Interfaces between T cells and APCs expressing elongated pMHC showed an increased intermembrane separation distance and less depletion of CD45. These results show the importance of the small size of the TCR–pMHC complex and support a role for size-based segregation of cell-surface molecules in TCR triggering.