Aspect Ratio Generator
There are two places that this addon is used: in the render properties, and the compositing node tree.
You'll find the panel at the bottom of the render property settings. Each of the buttons generates the correct resolution based on the selected aspect ratio and the width of your render resolution setting.
On the left shelf of the compositing window (if it's not visible, press "t" with your cursor over the window), find the "Aspect Ratio" category. There are a few presets, along with a custom aspect ratio generator. By clicking any of the buttons, a few nodes will be added before the composite. And if you change your mind and select a different ratio, it'll simply update the compositor - nothing will be re-added.
You can either using the preset aspect ratios, or enter the aspect ratio in decimal form in the "Custom Aspect Ratio" field. If you don't know the decimal form but have the proportions (the widescreen "16 : 9," for example), enter the width proportion divided by the height proportion, and let Blender calculate it for you (for the previous example, you'd type in "16 / 9")!
For the most accuracy with the custom aspect ratio, ensure that your custom ratio has at least 4 significant figures. This will ensure the accuracy of the resolution size. Otherwise, the resolution may be incorrect.