Gmail has never allowed you to sort mails. It is in fact impossible to see email size from the inbox view on the web app.
Google is making some changes. One of which has been rolled out today. You can now search for emails by size using the search box.
Google has also improved searching by date. Here are some examples:
Search for messages larger than the specified size in bytes
Example: size: 1000000 Meaning: All messages larger than 1MB (1,000,000 bytes) in size.
larger: and smaller:
Similar to size: but allows abbreviations for numbers
Example: larger:10M Meaning: All messages of at least 10M bytes (10,000,000 bytes) in size.