Hey @Jonathan5 - I tried searching for settings on shortening or adjusting the excerpt length for Previous Replies but, like you, I could only find post excerpt maxlength. I’d recommend submitting this under the Feature category since it could be something others are looking for and a great option to add along side the other setting. I know you can disable the excerpts from email notifications if you feel they are just too long, for the time being.
Here is a great article on finding those settings: How to control previous replies in emails