The width isn't the number of characters on a line - that wouldn't work with normal variable-spaced fonts, as the size of each character will vary.
The width parameter refers to the texture space width for the font. If your font is being displayed pixel perfect, it means the width is the number of pixels the where the line is wrapped at. The easiest way to size this is to physically move the widget in the interface. Otherwise, you will need to calculate the pixels you want to wrap at and enter that.