I think it uses print file QSYS/QSYSPRT. Unfortunately, so does a lot of other stuff. Depending on whether you’re willing to change this for everything relying on QSYSPRT for output, you might consider creating your own command that internally does OVRPRTF FILE(QSYSPRT) PAGESIZE(something new) before invoking CRTBNDRPG.
Hope that helps.
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