As it is done in content types for text area, it could be useful to add characters counter for text area in forms.
And also add in textarea settings the maxlenght parameter.
When maxlenght is set, characters counter could display :
nb characters / maxlenght

As an exemple, i am currently typing in a text area. I have typed 20 characters and i have set maxlength to 200. Characters counter zone should display:
Characters : 20/200