You’ve identified the Atlantic as the magazine you want to target.
Questions to ask:
How long are Atlantic articles?
How much are freelancers paid?
How do I submit a proposal?
To whom should I submit a proposal?
Your questions are all about what a writer needs to know to sell their article. The Writer’s Market is a reference work that all freelance writers should have. There is a fee but it is well worth having the “bible” for freelance writers. Here is the page about the Atlantic Monthly. Does it answer all your questions? Does it raise any new questions? (ie: What is a “kill fee” and why would you care?)