OK I understand you.
You know there are two types of reading in text from keyboard: inkey and inchar.
I think the second way should choose for your case, that reads only one character at once.
The base of this case, you must to know how wide is one character in your character set. That defines how many character fit into one line, assumed the set isn't proportional type.
So, with inchar you know when the width of typed in chars summed up is equal your line width. Then you open a new line.
I made kind of this computing in the past but not for type in text but for write out text, so I think it should work.

As an alternate solution you can try propose of 3run with WRAP, maybe that should work too.