Under Usage my Messages shows 1.1GB Used but I don't see any messages. How can I find and delete them?
I got this tip from http://www.igeeksblog.com/how-to-clear-message-imessage-cache-on-iphone-ipad/

You’ll need:


iBackupbot (Download)
Your iPhone/iPad
PC/Mac running latest version of iTunes


Follow these steps:


Backup your iPhone or iPad through iTunes. (Connect your iDevice to
the PC/Mac, under the devices in iTunes, right click on your device
and click on Backup Now; alternatively, you can also use the Back Up
option from the Summary tab)
Open iBackupbot
From the sidepane, navigate to System Files > Mediadomain > Library >
SMS > Attachments
All the folders within Attachments are your Message cache containing
all photos, video and media that you sent/received. Clear them out.
At its best, delete all folders.
Click on “Restore” so your iPhone is now restored to this backup.


Obviously, it doesn’t end here. If you have an iCloud backup already, you might have to flush that too. Go to Settings → iCloud → Storage &amp; Backup and clear out any backup that exists. You can create a new backup.