Not really enough info to answer question.
Save for web provides the smallest file as it stips away metadata.
How big the file would be would depend on the format and quality of the save. JPEG at 2 or 10?, PNG, TIFF, or ? As well as original format.
no, what I mean is it appears anything i open has a different size in photoshop than on disk?
im on a mac. if i open the info on a file in finder and see that a file is 12mb and then open it in photoshop and go to image size i see a whole different size?
why is this?
or is my photoshop broken?
If you were looking at the JPG on your hard drive then that is highly compressed. When it opens iin Photoshop you are looking at the raw data (as opposed to a Camera RAW file) which will typically be in the order of ten times the size of the JPG (according to quality settings when saved).
wait.... at the bottom of photoshop there is a number like Doc: 2.25M/12.5
the second number is the file size and the first number is the images size...
oh wow i never knew this
what is the difference between file size and image size?
Image size is best measured in pixels, such as 3000 pixels wide by 2000 high.
The OP needs to read the jpg wiki.
Neither of the numbers displayed in Photoshop is the file size.
Photoshop won't know the file size until you choose a file format and compression options, and the file actually gets saved to disk with all the compression applied.
Photoshop shows you the document sizes: flattened without compression, and the amount of memory that the document is using.