Virus-like particles (VLPs) have significant potential as both artificial vaccines and drug delivery systems. The ability to control their size has wide ranging utility, but achieving such controlled polymorphism using a single protein subunit is challenging as it requires altering VLP geometry. Here we achieve size control of MS2 bacteriophage VLPs via insertion of amino acid sequences in an external loop to shift its morphology to significantly larger forms. The resulting VLP size and geometry is controlled by altering the length and type of the insert used. Cryo-EM structures of the new VLPs, in combination with a kinetic model of their assembly, show that the abundance of wild type (T=3), T=4, D3 and D5 symmetrical VLPs can be controlled in this way. We propose a mechanism whereby the insert leads to a change in the dynamic behavior of the capsid protein dimer, affecting the interconversion between the symmetric and asymmetric conformers and thus determining VLP size and morphology.