Mojave doesn't remember window position/size for a specific application?
Here is the list of solutions:

Open a Finder window, hold down the option key and resize the window as you wish. Close all Finder windows and next time you open a new one it will remember the size.

Meanwhile, you can use an app like Stay to remember and automatically reset the position of any window. 

You might also look at window managers such as Magnet that allow easy manipulation of windows.