Hi Blerton,
Edit:
I see you mentioned list. My bad haha.
What you do is already on the right track by adjusting the "width" attribute. But, as you need to know that odoo's table is auto adjusted to fit the contents. If you have a very long title and very short postcode, it will auto adjust to try to fit it as "fair" as possible. Check the reference here

If you really insist to change the width. It might be too "forcing" and I afraid mess up other columns size. You can dive into the css or js files in the web module to know what's the width for each field types.
Hope this helps.
Previous reply:
I assume you are referring to this. It would be best if you could share your current code so we can work from there.

I would suggest you to follow this code as reference only and I only write the starting from the <form> tag only
<form string="Property Form">
<sheet>
<div class="oe_title">
<label for="name"/> <!-- This is the label for the name field: optional -->
<h1><field name="name" class="oe_inline"/></h1>
</div>
<group>
<group>
<field name="postcode"/>
<field name="available_from"/>
</group>
<group>
<field name="expected_price"/>
<field name="selling_price"/>
</group>
</group>
<!-- This is a notebook with description page: I skip this -->
<notebook></notebook>
</sheet>
</form>
Hope this helps.