If you have access to the Salesforce Campaigns module, then you can create a list using the members of a SF Campaign.
Any active campaign in your account will be available to use as a list. If you don't see a campaign listed as available in VR, then you should edit that campaign from within the Salesforce campaigns tab and ensure the "active" check box has been selected.
As with a lead and contact query, there are no restrictions on the number of campaign members you can add to a list, and any records marked as opt-out will be automatically suppressed from your list (ensuring you don't accidentally mail someone who has previously unsubscribed from your email).
You cannot use multiple campaigns to create a single list (the list and the campaign have a one-to-one relationship). If you want to send the same email to the members of multiple SF Campaigns, then create a list for each campaign. You can then choose to send the email to multiple lists (any duplicates between those lists will be automatically suppressed at the time of mailing).