Workaround for Email Signature Character Limit.

Before you begin, make sure your Salesforce username has the 'View Setup and Configuration' permission, read our documentation to find out more about permissions.

In Salesforce Classic: Signatures will be added to all personal emails and emails that use text templates.  

In Lightning Experience: Signatures will be added to all personal emails automatically, but inserting any template into email sent from non-case records will overwrite current email and signature.  Inserting a template into email sent from case records will not overwrite the current email and signature.