Evidently Quark 4.1 doesn't offer character count--there's no help topic on it and it doesn't appear in the index to the manual. (Actually this isn't much of a surprise, considering that word count is only available as a byproduct of a spelling check. Putting in a separate menu command for word count in a place people might think to look for it would taken what--a whole thirty minutes of programming time, maybe?)
It looks as if you're saying you also want to set the number of characters in a line, as distinct from the width of a text box or the margins of a paragraph. If that's the case, you can't do that. (And with a proportional font that's not how you'd want to do what I imagine you're trying to achieve anyway.)