Reduce high-quality photo size for texting and emailing

On a Mac, Photos File > Export dialogs would let you choose

• What encoding format to use (JPEG, HEIC, TIFF, or PNG) for exported copies
• What JPEG quality to use (Low (smallest size), Medium, High, or Maximum) for exported copies
• What photo size to use (small, medium, large, or custom) for exported copies
• Whether to strip location information from exported copies

This would not affect the photo in your library – only the copy that you were making "out in the Finder".

Export photos, videos, slideshows, and memories on Mac - Apple Support

For sending things directly from an iPhone,

• A Google search suggests that on the iPhone, if you share a photo directly to Mail, using the Share button within the Photos app, you will be prompted for an image size (Small, Medium, Large, or Actual Size).
• You can change the Settings for the iPhone's Messages app to "send smaller-sized previews of images so your recipients get them right away, before the full-resolution images arrive." That implies that you will actually send more data (the preview + the full-resolution image). See Take and edit photos or videos in Messages on iPhone - Apple Support .

When I start an email and begin to enter text, I get this

The paperclip means "add an attachment." Tapping that gets me to this menu

where I choose "Photo Library."

There I select pictures, tap the big blue arrow ↑, and the picture appears in my message. When I hit the "send" ↑ button, I get this menu.

Does this work for you? Where in the process does it differ?

I followed your instructions, but the prompts did not come up allowing for Small, Medium, Large or Actual Size. (I like your handle.☺️)