You can make this change without editing the game, in itch.io you have the wrong canvas size. You may want to change it to 400px by 800px instead of 300px by 600px in itch.io game settings. I actually skipped this game at first when I saw it a few days ago as I was unable to play it at all, but then I found a way to force resize the game to the correct size, then was able to play it. I guess if someone zoomed in on your game page that might wort of work too, but if someone didn't do either of those two things, they probably weren't actually able to play your game correctly, or at all.
I liked this game a lot actually. One thing is I think you may have enough content here that once it gets to the boss phase, it may be helpful to put a checkpoint there so the other part doesn't have to be redone again after dying. Even if you don't think you have much after that, I think it's enough that putting a checkpoint there may be better than starting over, since starting over may discourage some players from trying the game again.
Also, there's something exciting about the game, but having to do the first part may again get repetitive unless I felt like doing a challenge or something. I guess it may benefit from the default mode to be with checkpoints and without a score as it is now, and perhaps a challenge mode where it restarts you whenever you die and maybe even keeps a score, resetting it to zero when you die.
Either that, and/or maybe it benefits from additional improvements like having a few lives, perhaps some other things as well to improve it and keep the gameplay engaging.
However I really liked your game once I figured out how to be able to see the full area of the screen properly with a workaround and be able to play it.