Search Gmail messages by Size using the undocumented “size” operator
Wondering what email messages are eating up all the Gigabytes of space that you have in your Gmail mailbox? There are Google Scripts and external web apps that let you discover all the space-hogging email attachments but you don’t need them anymore.
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.
The size operator requires size in bytes (1 MB = 1048576 Bytes) but to make things a bit easier to remember, you can also think of the number as 1 followed by 6 zeros.
Update: As some have pointed in the comments, you may now specify the size in MB as well which makes it a lot easier. For instance, size:5MB (or even size:5M) will find all Gmail message that are 5 MB or greater in size.
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.