Signature Problem

Boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to nearly twice as long as a normal boot as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Wow, not sure how you are missing the Mail Preferences toolbar nor why the screen is so large. The window isn't resizable as far as I can tell. 11.5 should be readily available but there has been a massive CDN (Content Delivery Network) outage along with various DNS services not working that might prevent Mac's from downloading updates.

Try setting your DNS manually to the Google Public DNS as that worked for me this morning. 8.8.8.8 & 8.8.4.4 are the primary and secondary DNS servers that do not seem to be impacted by the Internet backbone outages that are ongoing.

Then check for Updates by System Preferences and Software Updates. 11.5 should appear.

I would guess that your Mail preferences are damaged. If after updating to 11.5 you could test with another Mac user account and see if the problem is not present. That would prove the issue is with some configuration files within ~/Library relating to the Mail App.

Looking at the files that are open for Mail reveals this one that might be the problem:

~/Library/Containers/com.apple.mail/Data/Library/Saved Application State/com.apple.mail.savedState

Try quitting Mail completely and then moving the Saved Application State folder listed above to somewhere else like the Desktop. Then start Mail back up. That should hopefully reset all the saved state for Mail. Things like window positions, X/Y coordinates and size of the windows, etc. There are multiple files in that folder that control remembering where all the windows are placed among other things. Those configuration files will be recreated when Mail starts up and they will be set to the default values as well.

Could be other similar configuration files but that is a logical first step.

Create and use email signatures in Mail on Mac

And as already asked, has the computer been updated to latest Big Sur 11.5 ?

Can you post a screen shot? What macOS version are you on?

Your details say 11.2. Did you upgrade to 11.4 or the very recent 11.5?