For general reference, the maximum attachment file size is 25 MB and the largest inserted picture size is 5 MB. This is useful information to have out there.

You don’t have to remove the ZDA file to shrink your attachment, though that will certainly work. If you go to the System Explorer and look at the Advanced tab, you can uncheck the box ‘Include Calculated Data in Session File’. This means that OS will have to re-run any analyses when you start it up again, which could well take a long time, but you can usually reduce the file size dramatically this way, particularly in NSC mode where you often have a lot of ray data.