From a user perspective, who wants to scroll thru 700,000 id's?
especially because the ssrs drop down does not allow type ahead, and the list is small
From a technical perspective, no platform, (asp.net, SSRS) will cleanly handle 700,000 items in a dropdownlist.
Your going to have to get creative.
Output the id's to a table, with other information that will help the user.
Then the report will drilldown when a user selects one of the id's?
Not sure what your trying to accomplish.