Let Photoshop Make The File Size Call
Oftentimes you have a target size you're trying to hit when creating Web graphics; for example, you're creating a Web banner and your file size limit is 32 K. If that's the case, and you know the target size, why not let Photoshop do all the work? Here's how: Under the File menu, go to Save for Web. In the Save for Web dialog, just to the right of the Settings pop-up menu, is a right-facing triangle. Click-and-hold it and when the pop-up menu appears, choose Optimize to File Size. In the dialog, enter the target file size you need your graphic to be and click OK to have Photoshop optimize the graphic to fit your target file size. If it doesn't matter to you whether it's a GIF or JPEG, choose Auto Select GIF/JPEG and Photoshop will "make the call."