Not Long ago Blogger Introduced the Embedded comment form and it was set to default 300px – 320px so Here is a tiny trick to entend the width of your comment box to make it look more neat.
Now as we always do login into your Blogger Dashboard
Dashboard > Layout > Edit HTML and check the “Expand Widget Templates” option.
Now as a precautionary measure please download your template so that you have an emergency backup if in case you create any errors
Now search for
Now here you can see here width='100%' so you have to change this value to something which will set properly as per your choice but please note that you have to keep the width few px lesser than your main warpper #main-wrapper. Its pretty easy so play on with the values and set it however you wish the width to be.