How to determine self.final_score_sz？

Hi, thank you for sharing the codes!
Given x_sz=255 and z_sz=127, the self.final_score_sz is set as 273. But how is 273 calculated？How can I determine self.final_score_sz in SiamVGGTracker for my dataset？
Also, hyparameters such as tot_stride and response_up are not clear. Could you provide breif explanations about them?
Your help would be much appreciated.