Pork loin vs pork roast? I've seen both boneless pork sirloin chops and pork sirloin roasts in the grocery store, for different prices.

From their names, I would think the chop is intended for grilling and the roast is intended for roasting, but if they're both cut from the sirloin, what's the difference between them besides the shape?
You've answered your own question, it's size and shape. You could buy a roast and slice it into chops and you'd get the same thing as if the butcher does it. That's if they are both pork sirloin, sometimes there can be confusion between sirloin and tenderloin, which are not the same thing.