Probably going to be load specific Catherine, ie a pallet with 40 X 25Kg bags of plastic resin stretch wrapped is fairly easy to move and keep under control, and stable.

An item of machinery on a pallet that weighs 750Kg, but is top heavy, might be unstable and unable to be moved safely by hand

@Catherine Ross
I think you are asking for an improbable number. My reasons for that include what @Steve H mentioned, the build of the person, capacity of the machine, shape of the load, size including height and width, the surface, plus more.
A brief online search indicates that in Australia, the US and the UK all regulations and procedures refer back to the manufacturer's guidelines. And there are variations from country to country.

My experience is that as soon as limits are declared, people paint themselves into a corner. Workers can over exert themselves or refuse to do the activity. Management can make unrealistic requests or create impractical processes.
As a suggestion, consider engagement with the other party and try to set general parameters based on common sense.

Just my ramblings.

I'm with KeithH, Catherine.
Instead of setting limits, for which there are far too many factors to arrive at anything meaningful, talk with those who have to use the equipment and ask for their input/experience, then work on guidance and making their work as easy and effective as possible, while using your H&S expertise (eg of ergonomics) to add to the conversation.
A question/discussion I've found really useful is to ask what has gone wrong in the past. If the environment is safe enough, you can get some amazing stories, not just of accidents, but also near misses. Great food for learning.

A separate point I'd make is to also look at the bigger picture. Are you incentivising staff on productivity or putting them under time pressure, implicitly or explicitly?

Another option could be look to replacing hand powered pallet lifters with battery powered versions, these would allow for variations in operators fitness/strength and size.

These take up a bit more space, wider aisle widths to compensate for their extra overall length, and for maneuverability, but the gain would be the additional weight they would allow to be moved- talk to your materials handling equipment supplier and see what they have in the way of options.