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The linewidth broadening caused by various physicochemical effects does limit the well-known advantage of ultrahigh color purity of metal halide perovskites (MHPs) for use in next-generation light-emitting diodes (LEDs). We have theoretically examined the quantum- and dielectric-confinement effects of a quantum dot (QD) on the degree of photoluminescence linewidth broadening. It is predicted that the linewidth (Δλ QC) is mainly contributed by the two opposing effects: (i) the linewidth broadening due to the repulsive kinetic energy of confined excitons () and (ii) the overall linewidth narrowing caused by the attractive Coulomb interaction (). It is shown that the relative contribution essentially remains at a constant value and is evaluated as , which is independent of the QD size and the chemical nature of semiconducting emitter. We have computed Δλ QC for various QD sizes of the prototypical MHP emitter, MAPbBr3, where MA denotes a methylammonium (CH3NH3) organic cation. The calculated results show that the linewidth broadening due to the quantum confinement (Δλ QC) increases rapidly beginning at the QD radius approximately equal to 6.5 nm but Δλ QC is less than 2 nm even at R = 1.5 nm. Thus, Δλ QC is much narrower than the linewidth caused by the exciton-LO phonon Fröhlich coupling (∼23.4 nm) which is known as the predominant mechanism of linewidth broadening in hybrid MHPs. Thus, the linewidth broadening due to the quantum confinement (Δλ QC) is not a risk factor in the realization of MHP-based ultrahigh-quality next-generation LEDs.