this is a very small issue, but I've noticed that on my game page settings for a webbuild when I specify the "view port" dimensions to match my resolution dimensions in the build of my game, I get a solid bar on the left and the top of the game window that isn't on the right and the bottom. I think this is some weird thing with the padding on the div of the container that holds the game window. To compensate I have to add units to the setting for "view port" dimensions to be greater than that of my game build. This allows there to be a good looking solid border around the entire window. It pushes the solid colored area right, and down when I do this.

The iframe dimensions will be exactly the value you provide. Some engines provide built in CSS that will remove any additional spacing, if not you'll need to make adjustments to the page CSS for the game you upload. There's no way for us to fix this from the outside without rewriting the embedded page's CSS. I think rendering the frame at the exactly at the specified dimensions is the best approach.