I need to save several files as 20mb jpgs. When I save an image in Photoshop CS5 at 20mb, then go to that saved file in my folder, it shows as a much smaller file. Is there a formula for hitting that target size? A ratio?
JPEG file size is a meaningless measure, except as an optimization criterion for bandwith/diskspace considerations.
Since JPEG is optimized to compress areas with little detail, while preserving fine detail, compression is dependent on the details in the image. There is no way you could produce a 20mb file from eg. a plain white image,
no matter the dimensions for any reasonable photo dimensions. On the other hand, a large image with a lot of fine detail will produce a large JPEG file when saved with a good quality setting.
So, in my opinion, you should review your requirements. There aren't a lot of reasons to require a specific JPEG file size, most often, you only need to respect a maximum size. To achive this, the "save for web" function offers a suitable option.