When I put a form into a main content area, I often find it looks awkward. In a typical content area of between 600-700px (like on this site), how wide should your form fields be? Contact forms or forms like this site do well with 100% width because they are expecting a fair amount of content, but for forms like login or registration, 100% width seems far too wide. Who is going to have a password that requires an input 700px wide? However, when I make narrower inputs, the remaining 200-300px gap to the right just feels awkward, especially when there is full-width content either above or below it.
Imagine that the following form is part of StackExchange with the current layout and width:

This gap feels awkward----------------------------------------------------------^
Does anyone else encounter this? How do you solve it elegantly?

