Instructions
Open your emulator by double-clicking the program's icon. Choose the "cartridge" you want to play and load the game. When the game is loaded and ready to play, you can alter the view.
Click the "Large Square" icon on the upper-right of the program window. Programs such as No$GBA have this simple option and it will make the program bigger, if not full-screen-size.
Click "View" in the toolbar. Select a "Full Screen" option if available, but if not, use the "Full Size" or similar view options. Keep trying "View" options until you find one that you like.
Drag the window larger. Put the cursor over the border of the program and drag the border wider and taller until you have it at a size you want. The program might have a maximum size, but you can pull until you reach it.