For the upload of a exe you gotta wait until the jam voting period ends, for the page settings under edit project go to embed options and change to fullscreen (this gives the listing page a play game button and after clicked it changes to fullscreen)
Minor edit: on android setting this feature turns it fullscreen, on web it takes the full screen except it will still show the top part of the browser (aka the tabs,searchbar,favorites) but a simple f11 makes it fullscreen
If you still prefer not fullscreen by default then consider setting the embeded in page dimensions Viewport dimensions to something smaller
Heres what one of my games has as default (Width 640 px × Height 360 px ) but that doesn't solve the problem for smaller devices tbh, so player would need to then click the fullscreen button in the corner for it to fit better