Could it be they’re both right? I mean, it’s possible that if you start rotating the model a few degrees on the z-axis, the measured dimensions on the platform might change.

Or, if it’s not a resin printer, you could always try the quick & dirty way: slice & start the print, pause it right after it extrudes the outline of the first layer (before it starts filling it in), and then measure the tolerances; resume if accurate, cancel if it’s off.

Blender is not CAD software and it's very difficult to get precise dimensions.

I would suggest learning to use proper CAD software. you can get a free 'hobbyist' version of autodesk fusion 360 that's fairly cut down. (it lacks things like simulation and substantially more advanced features.)

other options include solvespace and freecad which are both FOSS- Free cad is powerful, but really difficult to learn- with OpenFOAM integration to solve simulations (like force loads, and such,)

OnShape shape is like a shitty version of F360, IMO, with the caveat that it's free version also publishes your models publicly for anyone to see and use.

Solid Edge is also comparable to F360 (though I haven't used it,) and it has a "community" version that's similar to F360's cut down version.

This seems strangely just a little bit wrong.

As mentioned there's no units on STL files so the slicer assumes millimeters but warns you if it thinks the scaling is obviously not right.

My blender units are meters.

If I just export the default cube from blender (which is 2x2x2) and load it up in the slicer I get a prompt saying the units seem to defined in inches. It's assuming I can't possibly want to print a 2mm wide cube. Do I want to convert it to inches?

If I answer No, I get a perfect 2mm cube, good luck printing that. If I choose Yes, to convert it then I get a cube 50.8mm across. (2x 25.4mm) exactly 2 inches as expected.

Going back to blender, deleteing the default cube and creating another cube but scaled to 0.1m. This time exporting to STL on the export save dialog there is a scale setting: set this to 1000 (to convert from blender 1m units to Prusa 0.001m units). Now the STL is imported into the slicer with no fuss and is exactly a 100mm cube.

It seems I can't recreate your problem.

Blender has the concept of unit scaling iirc. You might try adding a simple cube mesh and exporting that to your slicer and see what it reports. If you still have a difference in size then I would blame something in your blender file. My typical workflow goes from fusion 360 to blender to slicer and haven’t run in to scaling issues (besides forgetting to set the units in fusion 360 export to mm).

firstly check that theres only one object, sometimes i've added objects without noticing in blender. if no make sure you didn't just rotate it which might make prusa slicer measure it wrong