Snapshots are created using sparse files, so it starts off around 64KB, and grows each time something writes to the source database (in this case, CRM). Blackbaud's performance testing team tested this by creating a snapshot then running performance workloads, and found that “In all cases, the space used for snapshot files is less than 1% of the database size.”  The upward limit of our testing was on  2.5 TB database, which resulted in a DB snapshot of 4.9 GB in size.