Before start, did you know what of scroll bot comment, just look at below picture
Already know it?proceed to the step!!
Design > Edit HTML > find the code <div class='comments' id='comments'>
Copy below code and replace to above code!!
<div class='comments' id='comments' style='overflow:auto; width:ancho;height:800px;'>Adjust word red color, that is for height the box !!
That All, THANKS