A-site cation composition is a useful lever in optimizing the photophysical properties and stability of metal halide perovskites (MHPs). Independent of this, straightforward preparative routes to MHP nanostructures that employ a single solid-state template with modest thermal requirements are also in demand. Here we employ both strategies in the fabrication and evaluation of luminescence properties of mixed formamidinium / cesium (CsxFA1-xPbBr3) and methylammonium / cesium (CsxMA1-xPbBr3) nanostructures formed within confining mesoporous silica of 4 nm and 7 nm average pore diameter. Use of such small pore oxide- terminated templates produce perovskite nanostructures in the strongly confined regime, with broadly tunable emission from green to sky blue (g). It is found that the smallest nanostructures that are formamidinium-rich exhibit the largest photoluminescence quantum efficiency values, but such values diminish by more than 50% in a 10-day period. In contrast, the same nanostructures formed within a 7 nm porous template retain their efficiency values over the same time window. The likely origins of this size-dependent behavior are discussed in terms of pore size-dependent capillary forces. Such routes may ultimately lead to improved light emitting diode designs composed of controlled quantum-confined perovskites of greater intrinsic stability than other emitters such as ligand-based colloidal nanocrystals.