How To Change Height and Width of blogger Comment Box

1.Log in to your dashboard--> layout- -> Edit HTML
2.Click on "Expand Widget Templates"
3.Scroll down to where you see this:

<iframe allowtransparency='true' class='blogger-iframe-colorize' frameborder='0' height='275' id='comment-editor' scrolling='no' src='' width='100%'/>

4.Now change width='100%' and height='275' as your choice.
5.Now save your template and you are done.