Accidents happen, and if one happened to your glass microwave tray, here's your opportunity to replace it!  I don't know the brand name of this tray or if it's just a generic replacement, but it is marked L18 close to the rim.  Also, please note the design at the center of the tray, which I understand is very important for making the tray revolve smoothly.

Diameter across the top:  approximately 12-1/4"
Diameter across the bottom:  approximately 11-1/2"
     (Does not include the curved rim.)

The tray is marked FOR MICROWAVE OVENS ONLY.  Please message with any questions.