Before You Go:
1. Go to Blogger Dashboard.
2. Click on your Blog Title.
3. Navigate to Template from left pane.
4. Click on Edit HTML button.
5. Find the below code in your template.
6. Find the immediate line next to the code you have found in above step.
7. In iframe tag you can find the attribute height. Use lower value for height attribute to reduce the height of comment form - i.e: height='250'
8. After changing height value, save your template and preview your blog. If the height feels uncomfortable, try another value for height attribute.