This site lists sensor size information for a camera and also cameras that have the same sensor size. First select the manufacturer, then select the model. The model's sensor size is given along with cameras that have similar sensor sizes in mm and pixels.
Select a model for your Sigma:
This site is a micro-site created using the Django Python Web Framework. The entire site is coded with a handful of code within a single html page.
Camera sensor sizes are from the Camera Sensor Size Database page.
View more micro-sites at microsites.xyzio.com