There's several ways to achieve this, just depending on what you want:
1. Assuming you're on v19, in the game's main file, open mkxp.json in notepad or any script editor. Scroll down and change "fixedAspectRatio" to false. "winResizable" must also be true, but I believe it's true by default. This will allow you to manually chance the game window by dragging the edges, or making it occupy the entire full screen. Do note that this will stretch out the pixels, and things will appear incorrectly sized while playing.
2. If you're not on v19, or you just don't want to use the above method, you can manually change the default aspect ratio (width x height) of the screen. By default, essentials comes at 512x384. Increasing the numbers in the Settings script section (the values are called SCREEN_WIDTH and SCREEN_HEIGHT), will make the screen size bigger. It will also allow the player to see farther while on the overworld - meaning you may see black on the edge of the map. You'll also have to update the UI menus.