I am becoming a big fan of Apple's Mail Drop to share pictures via email. I want to lower the threshold so that Mail Drop triggers with a smaller attachment size than it seems to currently. I found this article, but it appears to be out of date:
So, I took a look at the article and it seems that the particular value that the article goes about setting is minSizeKB
I am on High Sierra as I write this, Mail Drop is available on this OS so we should be on par as the service is available across any OS that supports iCloud.
In any event, in the terminal I entered the following:
defaults read com.apple.mail >> mail.txt
This reads the com.apple.mail file and outputs it to a file called mail.txt to my computer's home directory — you can name the file whatever you wish - we are doing this only to read the file, save it to disk, open it in text edit and search for minSizeKB and it occurs only once in the file for my version of Apple Mail.
The block of code looks like this and in my case it occurs way at the end of the file.
in the terminal window as this will output the contents of the .mail file to the screen where you may find the minSizeKB value at the very end of the terminal screen, as in the screenshot.
As you can see, on High Sierra's version of Apple Mail is it already set 20,000KB which is 20MB.
Now the article suggests setting this to 10,000KB which would be 10MB.
As the article states, this will trigger Mail Drop to be invoked if the attachment is 10MB or greater and this is your goal but this modification would only ever occur on the device for which you made this modification. The change will not cascade to other devices.
First you should read the com.apple.mail file and check what the value of minSizeKB is set to on your Apple Mail plist file for the version of MacOS that you are on. I suspect it's the same as this is widely available, works the same across all iCloud accounts etc. Apple doesn't appear to document this minimum though so the GUI has no slider to adjust the said property (minSizeKB). See, https://support.apple.com/en-us/HT203093
You may want to consider other ways of delivering attachments, outside of Apple's ecosystem. Here is a video by MacMost. Personally I use wetransfer.com. It's free but I don't know if the attachments are stored encrypted as they are on iCloud.