This is a really hard thing to do! The angle of the picture, the screens used in printing, how well you measure, even the lenses of the camera distort enough to make an accurate measurement close to impossible.
In theory, you would compare the size of something you know in the photograph (like the width of an oval inlay or pickup) and compare to the actual size and apply this result to the scale length. I just tried it several times, and sometimes the error is almost as big as the 2 inch difference you are trying to distinguish!
For an Alembic bass, the best thing is to find the serial number and have us check the build record.