How do you convert pixels to inches?

There is no fixed proportion, it depends how fine you want the print to be. eg. any good photo printer can work at 1200 DPI, so a 1200 pixel photo would be one inch across... Some work up to 4800 DPI or higher. The important things are how close you want to be able to look at it without seeing individual pixels, against how big you would like it to be. The lowest print resolution to consider is probably 100 DPI or so, giving a 12 x 8 inch print from a 1200 x 800 image. It would probably look a bit 'grainy' from close in. For comparison, a good 35mm colour film would have a pixel equivalent of roughly 3000 x 2000, and they are not too bad printed at 8x6 inches - but still make reasonable full poster size if you don't get too close.