(not the 10,000,000,000kms, but the 45kms one)
It is at least the size I had in the build window. I could not actually measure it, and even not see it, since it was a simple box. But this is checkable with a mesh, if it has some very small detail near its gravity center. We then can deduce how big the prim is from the size of the detail.
It is well know that some resizers can make prims much larger than sims. There was once a kilometres long prim found above mainland.
I used llSetLinkPrimitiveParamsFast( prim_number, [ PRIM_SIZE, NewSize] ), not the llSetScale(Whatever). Maybe the later is capped.
I think I would not like seeing prim sizes capped at 256 or something. There already is a wealth of sims surrounds, and they can be used for other purposes. I am currently preparing a surprise with them
Of course there is a potential for specific griefing, but it is as with many other things, we can live with it.