Find your dashed line, and then move the flags in the preview to widen or reduce the gap (or enter new values).
Also note the scale you are using as this will affect the appearance of your dashes.
The Dash and Gap of lines can be edited by going to Line Types... dialog box, selecting the line type, and changing these values. You may want to create a new line type (select one of the existing line types and click the "New..." button) so that your existing line types stay as they are. Also, note whether the line type you are editing is Scale with Plan or Scale Independent so you can control the result depending on the scale of the drawing.