Usually a document doesn't need to be in print size but may get scaled when printing via a custom "Scale" value. Unfortunately in particular for "Tiled" printing this field appears to be ignored by Affinity, though it would be required especially there, for instance to simulate a poster print. – Compare the recent 2022 posts in this 2019 thread:
It seems this "Scale + Tiled" issue is not logged yet – though I found two apparently unsolved topics (2019 / 2021) which could be related:
https://forum.affinity.serif.com/index.php?/topic/73605-print-bug-tiled-pages/
https://forum.affinity.serif.com/index.php?/topic/139281-affinity-designer-1921035-print-scaling-bug/