The example you have posted appears to be a newsletter? As such, there may be code embedded in the HTML which determines the width of the display: for example, the designer may have wanted to prevent the image at the top from being 'stretched' and distorted across your screen. If you know HTML you could view the source of the message to see if that is the case. Also you could try sending a message to yourself in HTML which is just text (no images) and which, when sent, runs across the width of the window: is it narrowed in the same way when it is displayed?

ALL my HTML email look like this and I have sent myself an HTML email and it does this, too, so it is something that Thunderbird is doing. As I said, I never had this problem until I went to El Capitan on my iMac, then everything that Thunderbird shows was messed up including my regular incoming email. The text on those was about 5pt. in size. I was able to semi-correct that with the "Theme Font and Size Changer" add-on but nothing has fixed the HTML emails.

I've read elsewhere in this forum about Mac users having problems like this, I think it's something to do with the fact that the Mac has many more pixels than other screen displays. You could try searching the forum answers for 'Mac' or 'El Capitan' to see if you can find those threads. In Thunderbird, the font size for composing HTML is set on the Preferences > Composition > General tab. The default is 'medium' I think and it's best to leave it like that because it suits most browsers and machines. The font size for reading HTML is on Preferences > Display > Formatting. You can set the default there (I have 14) and with the 'Advanced' button you can set a minimum size as well. I'm puzzled as to why Thunderbird restricts the display to the middle of your screen. I'm wondering if it's formatting the mail as though it were displayed on paper of a particular size. What do you have under File Menu > Page Setup?

Tried changing the page size and it had no effect. The font size is already set to 24 so that isn't the problem, either. Along with "Font and size changer" that's how I finally got my regular email to look somewhat normal and be able to read it. I'm starting to think it's a code problem with Thunderbird. Unless I can find a solution, I may be forced to use "Apple mail" instead. I've tried everything and nothing works.