Plugin DW2PDF doesn't support custom pagesize

[Juergen-aus-Zuendorf]

In the past, it was possible to set the parameter "pagesize" to custom values.
Example: "do=export_pdf&pagesize={216,303}"

But now, when processing the plugin, there comes an error message that the pagesize isn't allowed.

Best regards
Juergen

[Klap-in]

The format variable supports an array with the size in mm. https://mpdf.github.io/reference/mpdf-functions/construct.html
However, the pagesize url parameter assumes its value is a string. So probably your array is offered as string to mpdf library and not as array with numbers. Pull request, with proper sanitizing, is welcome.