I'd suggest you paste using "paste without formatting" (ctrl+shift+v, or on the Edit menu, or on the right-click context menu) so that the imported matter adopts and adapts to the resident default formatting.

Use of the Stationery add-on would allow you to define your styles in points, but I think it's misplaced effort. You have no say in what your correspondents use to view your messages, nor what personal preferences they apply, nor what limitations their devices assert. Trying to fix it with explicit point sizes is ultimately unhelpful to your correspondents. "Medium" means that they see the message at the size they want, not what you inflict on them.

Thank you for your prompt response. I didn't realize I could paste without formatting, so that is helpful.

There is something, though, that I don't understand in your answer:

You have no say in what your correspondents use to view your messages, nor what personal preferences they apply, nor what limitations their devices assert. Trying to fix it with explicit point sizes is ultimately unhelpful to your correspondents. "Medium" means that they see the message at the size they want, not what you inflict on them.

Are you saying that when I set the font size to "medium" (or "small" or "large"), it has no effect on what the recipient sees?

My usual response to queries about specific font sizes goes as follows; you send me a message, which I have been expecting and waiting for. When it arrives, I see it first on my phone, with its comparatively small screen, and in all likelihood, a limited choice of fonts, so possibly your message is displayed in a font other than what you had selected. I check it on my laptop which has something like a 14 inch screen. I take my laptop into a meeting where I can connect it to a projector and share your message with colleagues, where the screen might be well in excess of 1 m width.

In which of these three cases does your font size stipulation of 12 point actually make sense? Perhaps on the laptop.

But if you had used "medium" then in each case it would be shown at the relevant "normal" size, befitting the display it's being seen on.

I understand what you're saying and that makes sense. However, all I'm trying to do is to compose an email where the font and size of the text are consistent throughout the email I am writing. I suppose the paste without formatting will accomplish that.

So, despite the range of sizes of display devices, there is also the possibility that the user wishes to display text larger than the default, perhaps due to less than perfect eyesight.

I find myself that I need to set the display scaling in Windows to 125%, and even then, on a modern high resolution laptop, it's still too small to read comfortably. Making it any larger would result in some windows failing to fit inside the display. For ease of reading, I need to see it, in Windows, on a 23 inch display.

(Linux is a great deal more customizable and adjustable than Windows, but that's a whole different issue.  ;-) )

So if a message is set by the sender to use a specific size, my needs means it's unlikely to be shown as the sender intended.