How do I make photo different sizes in one email in Apple Mail?

This is a workaround, not a solution.

One way to achieve different sized images is to export your images (from Photos?) to preserve each original image entirely unmolested.

Open each exported image in Preview.

Once you've opened the image in Preview, Select Tools, Size.

Then select one of the pre-determined sizes, or create your own.

You can also specify the number of dots-per-inch of the edited image to be saved.

I typically export and resize my jpg original images to 6" width x 4" height and 300 dots per inch for printing to my inkjet printer on photo paper for casual snapshots.

Once you have edited the image, save it with a descriptive name, such as 'Black Monterey-6x4 300dpi.jpg'

Repeat the process for each image.

Preview can also save images in other file formats.

Now when you attach them in mail, attach all images as 'Full size', and every edited, saved image will be the precise size you chose.