We are using a Multi-line field in one of our ticket forms extensively and we have some agents checking those tickets regularly. The field is always completed with 15+ lines. Until now, the text box adapted on the screen depending on the number of lines (so it showed multiple lines). The field now shows only 2 lines of text.
I had a chat with Paul and he confirmed that if you add text/space to the already completed field, it would automatically adapt depending on the number of lines added in the field. However, it would be very useful to add a tickbox on the field itself when it is created, to 'allow' it to show the whole text once it's completed or not.