Tensorflow F-RCNN first stage input and output strides

Question

Trying to optimize performances on Tensorflow's faster_rcnn_resnet50 (from the model zoo), I'm currently working on understanding the full .config file they provide, and I'm having a hard time with the strides.

Here's the relevant part of the file, with bolded parts being things I'm unsure about:

image_resizer {
  keep_aspect_ratio_resizer {
    min_dimension: 1134
    max_dimension: 2016
  }
}
feature_extractor {
  type: "faster_rcnn_resnet50"
  first_stage_features_stride: 16
}
first_stage_anchor_generator {
  grid_anchor_generator {
    height: 28
    width: 31
    height_stride: 14
    width_stride: 15
    scales: 0.242
    scales: 0.621
    scales: 1.0
    scales: 1.739
    scales: 2.478
    aspect_ratios: 0.386
    aspect_ratios: 0.693
    aspect_ratios: 1.0
    aspect_ratios: 1.464
    aspect_ratios: 1.929
  }
}

What I would like to know is if I should be computing the feature map stride according to the anchors' dimensions and strides or if they're completely unrelated ? Also is there any restriction on the strides themselves, for example keeping them in powers of 2 like the original f-rcnn ?