What happens when the screen shot transfers to a file too small to send in an email?

When sending a screen shot, the Mail app should offer an Image size dropdown for your to select the size of the attached image.

Choose a larger size than the default or Actual Size to send it as is without any changes. do note most email severs do not support attachments larger than 10MB. You may need to select a smaller size.

I have a number of Draft emails ( using Mail app and on icloud.com ) saved in my dock, and all had a screenshot images in "actual size. " After a reboot, my MacBook Air 2013 ( Mojave ) changed all the photos in the draft emails to small . The draft email does not give me a dropdown option to change the image size. Any suggestion to get images back to actual size?

Its a pre-made shape in the Markup option in the Preview App.

You can change its color, thickness, and the text font, size, color and background color of the associated text box.