By default Crystal report viewer width is fixed according to it toolbar, but changes due to amount of data present in the report. And the width is not editable (as it is seen). Because of this reason if a crystal report has good amount of data then it goes past page width and disturbs the UI of that webpage.
One method to overcome this problem is hidden in the Crystal Report viewer itself, Viewer has a property named: "Best Fit page". Its value is true by default, change this value to false and then set the width of the report. Now you can change the width of the report to anything.
One drawback/functionality of turning this feature off is the horizontal and vertical scroll bar appears. Now this thing is good or bad, it depends on report scenarios.