Hello, Mike Lipay.
It's my understanding that you're unable to resize an image in the Mail app when composing an email. I may be able to help you with this. I would check to see if you're able to use this feature with another image. Some images may not have the option to resize:
Add attachments
By default, Mail inserts images at their actual size. If different sizes are available, you can choose one from the pop-up menu located on the right side of the message header.
You can also check out this article on how to resize an image:
Resize, rotate, or flip an image
You can resize or rotate an image opened in Preview to fit into a particular spot. You can also reduce the file size to save disk space or download an image more quickly.
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If the Markup toolbar isn’t showing, click the Show Markup Toolbar button , then click the Adjust Size button .
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Enter new values for width and height, or click the “Fit into” pop-up menu and choose a size.
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Resize the image by a percentage: Choose “percent” from the pop-up menu next to the Width and Height fields, then enter the percentage in those fields.
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Force the image to keep its original proportions: Select “Scale proportionally.” If you change either the width or height, the other value changes to maintain the proportions. If you choose a size from the “Fit into” pop-up menu, the image’s width or height may be smaller than the chosen size to maintain the proportions.
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Make the image’s dimensions smaller without losing any detail: Deselect “Resample image” before changing the width or height.
Tip: To resize multiple images at the same time, display the images in the same window, select them in that window’s sidebar, then choose Tools > Adjust Size.
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Rotate an image: Select the image, then click the Rotate button (or use the Touch Bar).
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Flip an image: Choose Tools > Flip Horizontal or Flip Vertical.
Tip: To rotate or flip multiple images at the same time, display the images in the same window, choose View > Thumbnails, select them in that window’s sidebar, then choose Tools > [Rotate or Flip].
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Choose Tools > Adjust Size.
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Select “Resample image.”
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Enter a smaller value in the Resolution field.
The new size is shown at the bottom.
Tip: To reduce the file size of multiple images at the same time, display the images in the same window, select them in that window’s sidebar, then choose Tools > Adjust Size.
Let me know if this helps.
Thank you for using Apple Support Communities.
Cheers!