Finetuning Fasterrcnn on custom object

Hi all,
I am currently going through the “train_faster_rcnn.py” code to finetune the model on my custom dataset. I wonder how can i modify the anchor ratios and scales values ? as my dataset has objects of various different sizes.
Also i am thinking to play with the “first_stage_max_proposals” , i.e. number of proposals passed from stage 1 to stage 2 ? how can i modify/fine tune these parameters.
Thanks
jalaj