How do i set default window size?

I would like to set window sizes of programs/prompts on my laptop to fullscreen, but after re-opening the program/prompt after sizing its window, it would go back to its default window size, which isn't fullscreen. How do I set the default windows size so that when all programs/prompts open their windows are at that size? Answer back as soon as possible, and thank you.

You say you sized the window but did you do this manually by dragging the window corners until the window took up the full screen size or did you simply click on the "Full Screen" square at the top right hand corner of the window? In order for Windows to keep/remember the window size you want you must do the former. Try it.