I was thinking to resize some large posters to a standard of ...x500 pixel
This is what the wiki says:
Is 182x256 really enough for a HDTV?"... and size of the picture to anything from 128x128 to 256x256 in pixel size, (the larger the image is the better it will look on a good display but at the same time large thumbnails loads slower). On a non-HDTV displays a image that is approx 100x140 is recomended for video/movies thumbnails (which is the size that is most common on IMDb), and approx 75x75 for music-albums (which is the default size on allmusic.com). For 720p/1080i HDTV displays a size of approx 182x256 for video/movies thumbnails, and approx 200x200 for music-albums (such music-albums can be found on allmusic.com). "