I need to create a triple hd canvas to export triple hd images with a 5760 x 1080 pixel size, is it possible to change the canvas size outiside of the already existing parameters?
You can do it via Scene > Scene Settings > Resolution tab and insert that resolution manually. You can save it as a new preset if you plan to use it again. Note that the Advanced and the Essentials editions of Harmony are limited, respectively, to 2K and 4K resolutions. The Premium edition allows you to custom much higher resolutions, beyond 8K. So you should need Premium to go beyond 4K.