How do I set the width of apple mail messages

That didn't work at all, I moved it all the way to the right and now I can't move it back

There is no way to get the cursor to the left of the scroll bar. You always just get an arrow. To the right of the scroll bar you can make the whole window smaller.

All that is visible is the summary of the messages

I finally called Apple Support about what I thought to be a memory leak. I asked the Support person to help me with that issue. She finally said she could only help me if I upgraded to Catalina (I can't because I am transitioning from a 32 bi accounting program who through the toel on upgrading to 64 bits very late). I told her I wouldn't accept her instruction. She got a senior support person who did what you were recommending in the first place.

I had done what you suggested but nothing improved, maybe because I didn't pull it far enough. I tried to pull from left of the scroll bar and that is where I got stuck.

Incidentally the memory leak is a much greater problem. See https://tidbits.com/2020/10/02/macos-10-14-6-mojave-supplemental-update-fixes-problems-with-an-updated-safari-14-0/

Any way thank you very much!