The length of the BLOB is a good indication of the size of the work object (case). I assuming you will be using your Dev / QA to get a ballpark on the size so that the staging / production can be sized appropriately.
To me it seems so complicated that it's perhaps easier to determine the space needed by actually observing how much space is used.
Here are some of the considerations for space needed:
blob space for work object and non-blob space for work object
blob and non-blob space for each assignment on that work object
space for all the attachments
How about measuring how much space is used by the db, then making some work and assignments and typical attachments, and measuring the space again, and using the observed value for your ballpark figure ?