Alison,
I don't know if you're replying to me or to Dave... Anyway, here's what I do more precisely. First of all, I'm talking of Apophysis. Let's say that I render a flame at 4800x3600 pixels; if I then open the image in Photoshop, it gives me an image of 66x50 inches at 72 ppi. In the "image size" menu, I just change the resolution to 300 ppi instead of 72, without resampling. The image stays at 4800x3600 pixels, but its actual size is reduced to 16x12 inches. I do that in the purpose of printing my images (300 ppi is correct for a good quality print).