The first evidence of boards for Rocket Lake may have arisen in a recent leak
The leaker momomo claims to have discovered fresh info on motherboards with as-yet unreleased Intel chipsets. Most of them are apparently compatible with Comet Lake-S, although 1 is described as having been made for the eventual successor to this series.
This Rocket Lake-S-based board is apparently of the micro-ATX form factor. The entire group of new boards was apparently briefly listed by Global American, a component manufacturer. The tipster momomo did have time to grab some information on these putative new boards, their model numbers included: for example the "new micro-ATX" may be known as the GA-IMB510.
These numbers may in turn refer to code-names for the architectures in question. Accordingly, Rocket Lake-S may correspond to Intel H510, whereas Comet Lake-S may be H410. Should momomo's information prove correct, those chipsets may have mini-ITX, thin mini-ITX and micro-ITX options.