I may have exaggerated the size a bit, looking at it now it is about 1 inch
long, and about the width of a penny, maybe a nickle, but the edges do curl
up _into_ the oven cavity, and arcing is a very good point.

If you are unsure, just go ahead and replace it.

Prolonged exposure to microwave energy of the amplitude generated by a
microwave oven could cause leukemia. - Reinhart