Find the target folder in Cornerstone and right-click on the folder name. Hold the Option key (to hide the username) and select "Copy URL without user".
Back in the terminal, run this command to create the checkout: svn co https://path/to/new/folder (using the value copied in the previous step for the URL).
If you are running this command from within a blank directory that should become the checkout, add a space and period at the end to target the current directory; otherwise, the checkout will be created in a new folder that matches the folder name of the last segment in the SVN path.
In most cases, you can checkout the job number folder that contains all of the various ad sizes, rather than a separate checkout for each ad size. You will still need to change directory to each ad size folder in order to run grunt compile assets, however.