Gmail supports an undocumented “size” search operator that you can use to 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 will list email 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 messages that are 5 MB or greater in size.
Combine the size search operator in Gmail with has:attachment and filename:ext operator and you have all the email messages that have attachments that are of a particular file type.
Gmail Size Search for Large Attachments
size:5mb has:attachment – All emails larger than 5 MB containing file attachments.
size:10mb has:attachment filename:pdf – Emails larger than 10 MB PDF attachments.
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.