Hi Dianna,
Thanks for your updates.
According to your description, I know that you encounter the issue when using Office 365 OWA via the Safari browser in the MacBook Pro.
In this situation, I would like to confirm more information below:
1. Which Safari version are you using?
2. If you use another browser, does the same issue occur?
In addition, you can upload a screenshot about "My email signature is truncated to 2 lines when I send a message from Office 365 mail on my MacBook Pro" here.
Any updates are appreciated.
Best regards,
Kid Yang