I presume you have resized that area by moving the splitter, but each time, upon opening, the modified splitter is not 'remembered'.

I also notice you are using a theme that is not any of the defaults.

Please restart Thunderbird in 'Safe Mode' and then open a new Write window. https://support.mozilla.org/en-US/kb/safe-mode-thunderbird

Please Report back on results.

Unfortunately the modified splitter IS remembered.

Opening in safe mode did not help

Help/Troubleshooting, Profile Folder, Open Folder, close TB, delete session.json. Restart TB and see if the pane retains its setting.

Thanks for trying to help, but still the pane is as big. Do not know why it is like that and why I cannot turn it off

Remove the wood theme, then restart TB. Does the pane still appear, even in safe mode?

yes, I have changed theme to standard and it still is that way. even after what you recommended

At the moment, I can't think of any other fix, but you could try a new profile and see if it works properly. Help/Troubleshooting, about:profiles, create a new profile and add an account. The original profile is unaffected and can be launched from about:profiles. If the new profile works OK, mail contacts etc. can be transferred from the original profile.

I'm wondering if a size has been remembered in the xulstore.json file

Help/Troubleshooting, Profile Folder, Open Folder, close TB delete both of these files:

• session.json
• xulstore.json

Restart TB and see if the pane retains its setting.