Mask R-CNN mask output shape

[sofiacatalan]

In the example Mask R-CNN model provided here, the mask output shape is shown as masks: ('nbox', 1, 28, 28).

Where does the 28, 28 come from? I'm trying to convert my own Mask R-CNN model from PyTorch, and for those dimensions, I get the length and width of my input image.

Model and opset version: Mask R-CNN, opset 11