InqScribe doesn't have a built-in way to limit characters, but you can get around it by doing this:
1. Open an InqScribe Transcript.
2. Select the Transcript->Transcript Settings... menu item.
3. Set the font to a fixed width (monospace) font, such as Courier
4. Type in "12345678901234567890123456" at the top of the screen
This gives you the width of 26 characters. If you have a different width, add or remove numbers.
5. You can then resize the width of the transcript window so that lines wrap right at the end of the last "6".
You'll still have to be careful about inadvertently wrapping lines, but it does give you a pretty good idea where the 26 character limit is.