You can specify the dimensions of your game's frame when you upload it. I recommend not making it too large though since it might be difficult for people with smaller screens to play. I recommend something below 1200 width. Likewise, a max height of 600 might be okay. If you need more space we do have a fullscreen button that can be used to make your game take up entire viewport. If you're planning on using that, you can set the initial size of the frame to be smaller.