Hi,
My standard response (to requirements like that) is: if the report generates such a large spool file, here is no way any human will be able to digest it all. Business users usually just want a brief summary and maybe a list of exceptions.
I think it would be best to make it an online report (using BAPIs / hitting standard tables) that users can run themselves over suitably small ranges. Then tweak the indexes to make sure it runs fast. Give them a link to the workflow log on each report line so they can fetch more detail if they want to.
But I know - if you're new to the project sometimes you just have to do what they tell you..
Snowing already..?
cheers
Paul