Using Adobe Dimension render output for commercial printing

1024 X 1024 square is the standard for 3/4 D work or to put it another way 1024 X 2 X 2 = 4096... your print size of 300ppi doesn't matter and the 12" sphere is the same rez as your 2 inch sphere example. What matters for material resolution is detail and the size you pick depends on both the model and that detail, for example a 2" sphare could get away with only 512 because its basic colour but a label with text wrapped around that sphare would need to be at least 1024 to allow the text to be focus correctly in the rendor so to answer your basic question of "how long is a pice of string" we say more detail = higher rez but it higher rez also = more work for your Cpu to do and therefore longer rendor times

p.s, 1024 X 1024 is the standard but many Windows users [including myself] work in 2048 X 2048 because A we have the Cpu power to do it and B when you rendor at 2048 you can then scale it down to 1024 for a smooth look