Outlook.com used on a desktop device will give the best experience for cleaning up your email. If you prefer to use a mobile device, Outlook.com accessed via a mobile browser offers a cleanup experience focused on helping you delete your largest emails. You can also use the Outlook app: tap the Filters icon in your inbox or another folder, then select Attachments to see the items that have the largest impact on your Microsoft storage. For more information, see How do I sort my inbox by unread, flagged/starred, or attachments?.
To see how much mailbox storage you're using and free up space, sign in to Outlook.com and go to Settings > Account > Storage. For more information, see Storage limits in Outlook.com.