double ScreenWidth;  //滿屏視窗寬度
double ScreenHeight; //滿屏視窗高度
double WindowWidth;  //WPF視窗寬度
double WindowHeight; //WPF視窗高度

ScreenWidth = SystemParameters.PrimaryScreenWidth;

ScreenHeight = SystemParameters.PrimaryScreenHeight;

WindowWidth = ScreenWidth/1.5;//這裡也可以設定你想要的任何值，如：800
WindowHeight = ScreenHeight*WindowWidth/ScreenWidth;//
 
這個就是根據螢幕的比例自動獲得高度

this.width = WindowWidth;
this.height = WindowHeight;

摘抄自：https://www.cnblogs.com/Caceolod/articles/2176325.html