Aaaa...O.K. I've got photoshop 6.0 and in the save dialogue box after you choose to save your image as a jpg has the information you are looking for (see attachment). I also have the option to 'save for web' and it gives you the size and even the download times on different connection types.

Calculating image size by pixels works for images like TIFFs that are not compressed. A jpg file compresses the information in an image to make the file smaller. As a simple example, if four pixels side by side are the same color, a jpg only saves it as one piece of information instead of four. A tiff would save four pieces of information regardless.