Painfully, they're both called depthMapSize internally. The parameter "Map Size" is as @adp001 says, while the depthMapSize attribute is not exposed to python directly.

However, you could create a simple library shadowMapSize.lt2 file with a script, containing:

{
version
    {
    number 6//11
    build 40391
    }

light spotLight 1
    {
    depthMapSize 1024 
    }
}

where spotlight 1 is whatever the internal name of the selected light is, then have the script do:

poser.Scene().LoadLibraryLight('path_to_file:shadowMapSize.lt2')

I have a render running, so I can't test this immediately, but the only caveat that I can think of with that method is that Poser likes to remove all the existing lights whenever you apply a light pose from the library, so you may end up with only that light and no parameters set for it, which wouldn't be nice. Then you'd have to resort to saving all the lights to a file, parsing the file and changing the depthMapSize then loading the light file again. Blearghh!

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