Gmail supports an undocumented “size” search operator that will let you quickly find all the big emails in the mailbox whose size exceeds a particular threshold. The search syntax is:
size:[size in bytes]
To give a real example, a Gmail search query like size:5242880 will list all big emails who size is 5 MB or greater while size:10485760 list messages that are bigger than 10 MB.
You may specify the size in MB as well which makes it a lot easier. For instance,size:5MB (or say size:5M) will find all Gmail message that are 5 MB or greater in size.
In addition to size, you can use search operators like “larger”, “larger_than”, “smaller” and “smaller_than” to find files of any particular size. A combination of these size search operators is possible too. For instance, the following Gmail search query will find all emails with attachment sizes between 5 MB and 8 MB.
Once you have spotted the large emails attachments that are taking too much space, apply the #googleDrive label to directly save those attachments to Google Drive before deleting them from your Gmail mailbox.