1. Login to Blogger Dashbord and go to Design >> Edit HTML. Click on than Expand Widget Templates.
2. Search
comment-editor
You will get following code
<a expr:href='data:post.commentFormIframeSrc' id='comment-editor-src'/>
<iframe allowtransparency='true' class='blogger-iframe-colorize blogger-comment-from-post' frameborder='0' height='410' id='comment-editor' name='comment-editor' src='' width='100%'/>
3. change the values of height and width according to your values. And save your template.
Notes: Dear all before doing this you must backup your full template