
How to Change Blogger Comment Form Size?
- Go to Your Blogger Account Dashboard > Design > Edit HTML
- Check "Expand Widget Templates" box

- Search This code (Hint: press CTRL+F keys from your keyboard and copy-paste this code)
<a expr:href='data:post.commentFormIframeSrc' id='comment-editor-src'/>
- Just below searched code, you will get an iframe code which will look like
<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='' style='display: none' width='100%'/>
<b:else/>
<h4 id='comment-post-message'><data:postCommentMsg/></h4>
<p><data:blogCommentMessage/></p>
<data:blogTeamBlogMessage/>
<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%'/>
<iframe allowtransparency='true' class='blogger-iframe-colorize blogger-comment-from-post' frameborder='0' height='410' id='comment-editor' name='comment-editor' src='' style='display: none' width='100%'/>
<b:else/>
<h4 id='comment-post-message'><data:postCommentMsg/></h4>
<p><data:blogCommentMessage/></p>
<data:blogTeamBlogMessage/>
<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%'/>

- Change the size from 100% to any fix size such as 500px, 600px etc
- Save Your Template and its Done!