Knowing what size you are going to make your paintings before you begin is tremendously helpful. By knowing this, you can now be exact with your composition.
There is something called Aspect Ratio most commonly used to determine screen dimensions in broadcast media (TV, cinema films, YouTube...). Aspect Ratio is that notice you used to see on VHS tapes stating 'this film has been formatted to fit your screen'.
The aspect ratio of the drawing for the painting above (inside the grey rectangle) is 20:16 = 20"x16" (Examples of aspect ratios of different dimensions, but the same proportions would be 5:4, 10:8, and 1:0.8)
By knowing this, you can create a shape in those dimensions and account for all the space used in your composition. The advantage is: when you blow up the drawing (via photocopier to transfer it to your painting surface) you won't have negative space in your painting surface that you did not account for. (see below)
I like scanning the drawing, bringing it into a digital coloring program and placing a shape over it in the dimensions of the final painting. You can always move around that shape and angle it so the drawing fits in the most interesting way.
Then, print out that image at a manageable size to tighten up the drawing (above). Although the shape is less than the final dimensions, you know that when you scale it up, it will be EXACTLY the size you want. There are many print places where you can do large format printing (anything beyond 12"x18"). You can get your image printed out at the scale you want (in this case, 20"x16").