Measuring powdered sugar
My best guess would be one pound, since it's still often sold in stores in 1lb boxes, regardless of brand:

 

You can see plenty more like that if you do a Google image search for [powdered sugar box].

As others have pointed out, measuring by volume is inaccurate (one pound could be 3.5 to 4.5 cups), so you'd be best off using a scale, or perhaps measuring your two pound bag into two equal volumes.

That said, if your recipe can tolerate being off by a couple ounces (around half a cup), you could use four cups for a one-pound box.