Here we will mention the name of paper format 

<field name="name">Custom Paper Format</field>

Here we will provide a predefined format (A0 to A9, B0 to B10, Legal, Letter, Tabloid etc.) or use a custom one such as A4 by default.

<field name="format">custom</field>

Here we will provide the page height and width in Millimeters.

<field name="page_height">297</field> 
<field name="page_width">210</field>

For orientation field we can use Portrait or Landscape.

<field name="orientation">Landscape</field>

Here in the margin fields we can define our margin dimensions in Millimeters.

<field name="margin_top">15</field>
<field name="margin_bottom">10</field>
<field name="margin_left">10</field>
<field name="margin_right">10</field>

Here we can use a boolean expression to display a header line in your page.

<field name="header_line" eval="False"/>

<field name="header_spacing">3</field>

<field name="dpi">90</field>

<report id="your_custom_report_id" 
         string="Name of the report" 
         model="your model" 
         report_type="qweb-pdf" 
         file="your_module_name.your_custom_report_template" 
         name="your_module_name.your_custom_report_template" 
         paperformat="your_module_name.your_custom_paperformat_id"/>

These are the steps to add a custom paper format to a custom qweb report in Odoo.