macOS Hints: Set Mail Attachment Size
Since the first version of macOS, Apple's own mail program has been built in. Mail offers many useful functions for receiving and sending e-mail messages.
With the following steps you can make sure that Mail does not send the attachments too small:
After you have written your new e-mail message in the mail window, add at least one image as an attachment.
Now a new line "E-mail size" appears under the line "From". On the right side of the line you will also find the option "Image size".
Select "Original Size" from the drop-down menu.
This will send the original image in its full size to the recipient. Otherwise, it is possible that the image may be too small and illegible for the recipient.
More about mail attachments can be found directly on the Apple Website.
As long as you do not change it, screenshots are automatically sent in the same size as last set.