Hey Omar,
I do not believe Mkto monitors your database size on a daily basis. That being said, I would import the list into a static list. Then, send your email to that list and monitor the responses. Those who unsubscribe, or respond with "no thanks," or don't respond, or don't even open/click the email after a week, you can delete those folks from your database. Those you have a positive response from can be marked however you please so that they are designated as Leads. I'm not sure how you have things set up, but make sure to place everyone in this list in a certain status that will not add them to any other initiatives you have going on.