I'd be tempted to try the broody to make things easier on yourself (no need to provide heat, no need to integrate) but that's up to you. I've never had a bird go broody at the right time to try it.
You won't need them to stay in there 5-6 weeks. With your temperatures plan on weaning them off at 3 weeks, maybe 4 at latest.
If you're dead set on keeping them locked up for 6 weeks, then I'd give them around 12 sq ft. But if you're considering early integration, better to have them out sooner rather than later, in order to use their small size to your advantage. How I handle early integration: https://www.backyardchickens.com/articles/short-on-time-recycle-a-prefab-brooder.73985/
How big of a brooder do you think for 8 chicks to stay in for 5 or 6 weeks?
You won't need them to stay in there 5-6 weeks. With your temperatures plan on weaning them off at 3 weeks, maybe 4 at latest.
If you're dead set on keeping them locked up for 6 weeks, then I'd give them around 12 sq ft. But if you're considering early integration, better to have them out sooner rather than later, in order to use their small size to your advantage. How I handle early integration: https://www.backyardchickens.com/articles/short-on-time-recycle-a-prefab-brooder.73985/