As noted, Apple has said nothing about any iMacs other than that the Intel models will be replaced with M-series SoCs.
But there has been plenty of ancillary data released from multiple sources, some with very high "hit rates" on their claims, that the M-series iMacs would be larger than their Intel predecessors. We have had rumors of a 23-24" M1 iMac since early 2020 and rumors that there would be an iMac larger than 27" started gaining traction in January 2021 with l0vetodream and Bloomberg (though I now believe the J456 and J457 model numbers Bloomberg referenced are for the 2-port and 4-port 24" iMac).
The only claim I can recall about there being a direct 27" replacement for the mid-2020 iMac 5K was Ming Chi Kuo, who in March 2020 said Apple was working on a new iMac Pro that would have a 27" miniLED display that he said would launch in late 2020 alongside a 24" M1 iMac and a miniLED 12.9" iPad Pro. (Assumption at the time was the iMac Pro would be an Intel-powered model using the new W-Series Xeons Intel had released.)