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Glen,
Great project and very time consuming.
One of the brightest PN is NGC 3242 (Ghost of Jupiter), size 25". The Blue Planetary (NGC 3918) is about 12" diameter. You can't exclude these.
Don't forget the eta Carinae nebula (NGC 3372)!
For all objects, I'd be inclined to go for observed brightness rather than size as a criteria. As you have mentioned Vmag is not a great indicator for large objects so Surface Brightness will come into play.
Regards, Rob
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