it's really not the worst thing you could make for a first jam with entirely new software, and as for the fullscreen thing, i'm not entirely sure what's causing it but i know what might help (here it is on godot 4.2 but its probably the same in like every 4.x version):
go to "project settings" from the "project" dropdown and find window settings under display
here you can find where to change the width and height (here i have it set to 1280 by 720), and what i think will help your particular problem is under stretch you can set it to stretch the whole viewport when it scales up rather than just the canvas, which should avoid having just like a gray rectangle on the side, sort of forcing your game into a certain aspect ratio.
i hope that helps! and i encourage you to keep making games and learning!