I think I’ve calculated the size and resolution of the new big iMac

[Apple Knowledge Navigator]

...I’d just like to say that I think there is virtually zero chance Apple replaces the 27” iMac with a 32” 6k iMac. It just makes no sense. The 6k XDR will remain their premium display and it will remain the biggest… and most expensive. The 5k iMac is a consumer machine and costs far less. They’re not going to match the size and resolution of their $5000 display. Not gonna happen...

...I think the next big iMac will be have a screen size of around 28.7” and a resolution about 5480x3050...

Agreed. Like the new 24" iMac, I think Apple will have aimed to keep the chassis size of the 'larger' iMac roughly the same, but through reducing the thickness of the bezels, will increase the display size to around 28-9".

Mini-LED is a possibility on at least one model - perhaps the higher tier product. My theory with this is that the chassis will already be slightly thicker in order to accommodate USB-A and SD card ports and larger fans for the SoC, so they could use the additional space behind the display for the mini-LED components.

And keeping in mind that this machine will likely have a silver and darker colour option, I think they'll just call it the iMac Pro, which is level with the rest of their product lines (larger, more capable devices).