30 cm is about 12 inches, and 40 cm is about 16 inches, so to produce a 300 PPI image of that size you'd want to render at 3,600 by 4,800 pixels. You'd work out the size of your poster the same way (though for something that is going to be viewed on the wall you may well not need 300PPI, which would reduce the image size). However, you should bear in mind the limits of your raw materials - there's not much point in producing a high-reoslution image if some elements of your scene have only a 500 pixel square section of texture stretched across the whole image area, since that will be grossly pixelated anyway.