On your itch game page you may have a link on top left "Edit Game", near "Analytics" and "Widget" (not 100% sure of the words, I'm french and have itch in french too). On this "Edit" page, you can scroll down after the "Online files" section (where you uploaded your zip file) and you can set the window size here. 1280*720 is a good value I think, as you allow players to also switch to fullscreen !