Unfortunately, Fusion 360 only allows a maximum render size of 4000px X 4000px, regardless of license type (free student/hobbyist or commercial). This has been brought up in the past, but there hasn't been enough interest to make this a priority. I would suggest making a post on the "Idea Station" to see if there is now more interest from the user base.
If you need to create a render larger than 4000 pixels then you can export your Fusion 360 file and import it into another program, such as Autodesk Maya. Maya has no limitation on rendering size. If you're a student you can use Maya for free. They also offer a 30 day free trial.
Cheers,
Kevin
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