speaking of overkill: at 300 dpi, a standard 4R (6in x 4in) photo only needs a resolution of 1800x1200, or roughly 2 megapixels. but the wide majority of modern camera sensors and analog 35mm film well exceed that standard; you can blow up the images far larger with no detectible loss in fidelity
but in the interests of keeping things nice and distributable and affordable i will print photos mostly in 4R