You can create a custom paper format for your reports, and then set it to a report, for example.

<record id="paperformat_euro_lowmargin" model="report.paperformat">
    <field name="name">European A4 low margin</field>
    <field name="default" eval="True"/>
    <field name="format">A4</field>
    <field name="page_height">0</field>
    <field name="page_width">0</field>
    <field name="orientation">Portrait</field>
    <field name="margin_top">5</field>
    <field name="margin_bottom">5</field>
    <field name="margin_left">5</field>
    <field name="margin_right">5</field>
    <field name="header_line" eval="False"/>
    <field name="header_spacing">0</field>
    <field name="dpi">80</field>
</record>
<report
    id="report_event_registration_badge"
    model="event.registration"
    string="Registration Badge"
    report_type="qweb-pdf"
    name="event.event_registration_report_template_badge"
    file="event.event_registration_report_template_badge"
    paperformat="event.paperformat_euro_lowmargin"/>