An <input>'s width is generated from its size attribute. The default size is what's driving the auto width.
You could try width:100% as illustrated in my example below.
Doesn't fill width:
<form action='' method='post' style='width:200px;background:khaki'>
<input style='width:auto' />
</form>
Fills width:
<form action='' method='post' style='width:200px;background:khaki'>
<input style='width:100%' />
</form>
Smaller size, smaller width:
<form action='' method='post' style='width:200px;background:khaki'>
<input size='5' />
</form>
UPDATE
Here's the best I could do after a few minutes. It's 1px off in FF, Chrome, and Safari, and perfect in IE. (The problem is #^&* IE applies borders differently than everyone else so it's not consistent.)
<div style='padding:30px;width:200px;background:red'>
<form action='' method='post' style='width:200px;background:blue;padding:3px'>
<input size='' style='width:100%;margin:-3px;border:2px inset #eee' />
<br /><br />
<input size='' style='width:100%' />
</form>
</div>
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