Prototype calorimeters based on LYSO and LaBr3:Ce have been simulated, assuming different crystal sizes, from the currently available ones with a length of 16 cm and a diameter of 7 cm to an ultimate size of about 10 Molière radii and 15 radiation lengths. These crystals are coupled to O(100) silicon photomultipliers for the available crystals. The response to 55 {MeV} photons was studied, a typical energy to be expected in charged lepton flavour violation experiments with muon decays. Simulations of the available size crystals show an energy resolution around 1.7% for LYSO and 2.5% for LaBr₃:Ce as well as a timing resolution around 30 ps and a position resolution of O(5 mm) for both crystals. The simulations for the largest crystals indicated resolutions significantly below 1%, with clearly better values for LaBr₃:Ce. Such performances pose these future calorimeters at the technology forefront and make them eligible candidates for upcoming experiments.