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Make sure you're using a 64 bit build of QGIS. The limitation on exported composer sizes/DPI is much higher on a 64 bit build.
You're going to be doing accuracy assessment. You want to bring the data for validation (Google Earth) into ArcMap. Then you can "truth" the classification for each of your random points in a shapefile. Afterwards you can sample those same points in your classified image and compute your error matrix.
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