You can create a custom paperformat like this in your report.xml

<record id="my_custom_format" model="report.paperformat">

     <field name="name">Item Label</field>

     <field name="default" eval="True"/>

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

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

     <field name="page_width">254</field>

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

     <field name="margin_top">0</field>

     <field name="margin_bottom">0</field>

     <field name="margin_left">0</field>

     <field name="margin_right">0</field>

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

     <field name="header_spacing">0</field>

    <field name="dpi">80</field>

</record>

then link it with your report

<record id="id_of_your_report" model="ir.actions.report.xml">

      <field name="paperformat_id" ref="YourModule.my_custom_format" />

</record>

Thanks zbik for your reply but sorry, I don't get it

Here is my xml paper format content:

<report

     id="MyModule_paper_report"

     string="MyModule"

     model="MyModule.paper_report"

     report_type="qweb-pdf"

    name="MyModule.paper_report"

    file="MyModule.paper_report"

/>

I tried to do it in the xml instead the code (your links) but without success neither. In fact I don't know what to replace in your code example.

By setting string="Invoices" (as above in xml) is doesn't work
When I check In Settings/Technical/Reports/Paper Format/ MyModule => Displays "Invoices" but not working when I print one invoice

Thanks to all.

What I was looking for was : <record id="account.account_invoices"

Done in xml (instead python code) because much simpler.

=> When I check In Settings/Technical/Reports/Paper Format/ MyModule => Displays "Invoices" and works when I print one invoice.

May be I was not clear enough in the description. Sorry for that.

See, for example:

https://www.odoo.com/forum/help-1/question/how-could-i-use-t-if-in-a-qweb-report-to-test-the-paper-format-77501 

https://www.odoo.com/forum/help-1/question/how-to-instantiate-a-paperformat-from-the-model-report-paperformat-77536