#C#, 85 Bytes
()=>System.Console.Write(System.Console.WindowWidth+"x"+System.Console.WindowHeight);
If you want a whole application code, we'd have 115 Bytes:
using System;static class Program{static void Main()=>Console.Write(Console.WindowWidth+"x"+Console.WindowHeight);}
However you could mean the Buffer size as well, we'd have 85 Bytes again
()=>System.Console.Write(System.Console.BufferWidth+"x"+System.Console.BufferHeight);
Or as a whole application code (obviously 115 Bytes again):
using System;static class Program{static void Main()=>Console.Write(Console.BufferWidth+"x"+Console.BufferHeight);}