OK
the error message because of the order is odd, however that could be because of \ acting as some escape but in your original post you said simply using “shrink” gave a problem thus I have to suspect there is some driver communication issues however you can manage to print both sizes
Your prime issue is still down to the printer only using A4 by default ? (its windows default setting?)
In such cases I find the sure fire way to settle this is to have multiple copies of the driver with names I like
one may be “A4printerLand” with default settings for Landscape A4 paper another “A3printerPort” with default settings for Portrait A3 posters sending the correct input to each should work around any driver mismatch
It is important that you check in each case the settings for scaling so in the windows driver there should be preferences for page size rotation and scaling either as % or similar to [shrink | fit | none]