You can do that using Aeroo Reports and adjusting page sizes directly on OpenOffice.

Works just as expected!

Should be possible by modifying the custom_rml.xsl file like:

    <xsl:template name="rml">
        <document filename="example.pdf">
            <template>
                <pageTemplate id="sticker" pageSize="(3cm,5cm)" >
                    <frame id="main" x1="0cm" y1="5cm" width="3cm" height="5cm"/>
                </pageTemplate>
                <pageTemplate id="a4_Portrait" pageSize="(21cm,29.7cm)" leftMargin="2.0cm" rightMargin="2.0cm" topMargin="2.0cm" bottomMargin="2.0cm" >
                    <frame id="main" x1="2cm" y1="25cm" width="19.0cm" height="25cm"/>
                </pageTemplate>             
            </template>
...

and then calling the template in your_report.xsl file:

<xsl:template name="story">
     <xsl:attribute name="firstPageTemplate">a4_portrait</xsl:attribute>
</xsl:template>
...
<setNextTemplate>
    <xsl:attribute name="name">sticker</xsl:attribute>
</setNextTemplate>

However, in my case this is working for the Frame setting but not for the pageSize! But according to the RML doc this should work as well for the pageSize setting. What can be wrong?