for this to work you will need 2 mods (apart from planbuild, obviously) . 'valheimplus' (to turn off structural integrity) and the other is 'precise rotation' (to set the rotation to 32). i could not upload the entire colosseum in 1 go because planbuild cant handle it, so this is 1/32 of the build with just repeats this 1 slice, place this 1 slice, rotate 1/32 and place again. keep doing it 32 times and you are done!

Full size Rome Colosseum. 9024 actual chairs you can sit on in the seating areas, 100 entrances, 96 staircases, curved/ arched ceilings, 214 meters wide, 50 meters tall