How to determine anchor sizes for RetinaNet model?

I posted it on stack overflow but got no replies so i am posting it here. I am trying to implement a RetinaNet model in pytorch for my custom dataset, however, i am little confused on how some of the hyper-parameters are chosen. For example, the model uses anchor size of [32, 64, 128, 256, 512] , anchor ratio of [0.5, 1, 2] and anchor scale of [1, 1.25, 1.58] , but how do we determine these numbers for our own datasets. What are some of the ways we can go about when selecting param for the anchor box generation part ?