My personal preference for picture size for brickshelf pictures is between 640x480 and 800x600. This is a good size for quality while leaving the viewer a short time to download the image. AFAIK, something like 70-80% of people have their computer set to 800x600 resolutions, so said pictures would take up their entire screen.
As for stories, I typically go with the largest dimention, on average, not to exceed 500 pixels, unless the ratio to height and width is huge (like a 800x50 picture).
Give a man a fire and he'll be warm for a day.
Set a man on fire and he'll be warm for the rest of his life.