As the previous responses explain, Save for Web is specifically designed to make small images for use on web pages. To save larger resolution files, resize the image first, then use the normal File -> Save As menu item and select JPEG as the file type.
You cannot realistically force the dimensions and file size of an arbitrary image to meet some agency's specs. The file size depends on how complex the image is - a 10,000 pixel all-white JPEG saved at maximum quality will be a tiny file.
