Do you mean that setting the project's stretch mode to canvas_items did not give you the results that you wanted? If that is the case, you could look into using stretch scale. Or if it is just the font size that you want to adjust, you could use the size available to the Label to determine what size to make the font.
I'm not sure if any of that is helpful to you or if I fully understand the issue that you encountered, but if there is some way I can help, feel free to reach out to me here or on Discord (my username is Mjb117). I'd be happy to discuss any of this further.