You need to explain more about what you need to do. Are all short paragraphs to one style, and anything longer a different style, or do you want to apply a character style up to a certain point in a longer paragraph?
No I don't want to apply a style up to a certain point in a paragraph. I want to apply a paragraph style to all paragraphs that are 45 characters or less. These paragraphs are headings, and the longest one is 45 characters.
I think this will work:
In the change formatting choose your header paragraph style
Works perfectly. Thank you!