If the baggage is bigger than that which can or should be safely stored in the overhead luggage racks yet people are using them anyway, then the railways do need to look at luggage rules.
The real issue on the IETs is the lack of luggage space though. The LNER ones are better since they took out the seats behind the door pockets and replaced them with luggage racks, but even so.
Perhaps, we need a size that says "suitable for trains" in the same way we have for bags that people might want to take on trains? There does seem to be this idea that the large boxes on wheels that would be in the hold of an aircraft and in which you could transport a small body are suitable for a train.
I usually travel for an overnight stay, so my bag fits (just) in the space in front of my seat in Standard, more awkwardly in First because of the table leg. But I've also seen my fair share of bags dangling precariously off the overhead racks.
I do remember being on an MML set from Sheffield once, in First; someone wheeled a 'regular' size suitcase down to the luggage rack in the centre of the carriage... and found it was too tall for the lower section. Suitcases grew between the 1970s and 2010s

It's a variation on Parkinson's Generalised Law, which states that everything expands to fill the space available to it. Prams are the same; they were all slim buggies until buses were made low-floor and accessible, so they expanded to fill the space available.