I don't know if validation rules work in IWP. I think so. If so:
Length (Self) > 59
would require the user to enter at lest 60 characters.
Otherwise, you may need to set up some sort of scripted check using this same basic calculation, but with the field name in place of 'self', folded into a script that they have to perform when clicking a button such as a submit button on the layout.