The exact file size is not directly relevant.
* If you want to keep/save the full image details, you must shoot in RAW and save the RAW file.
* If you prefer to shoot JPEG, the Mpixel size of the shot is important. I suggest that you use JPEG 12 Mp, to minimise any distortion/recomposition of the image by the JPEG 'routine'.
With my K-7 (14 Mp), i tried all combinations of JPEGs from 14 Mp to 2 Mp and from 4 stars [****] Premium to 1 star [*] Good. My preference is 14 Mp 2 stars [**] which gives me (a) some excellent resolution and (b) file sizes smal enough (for my use). When I want more details, I shoot RAW+.
Hope that the comment will help...