What are the best practices for content to scale properly within any browser size? I don't see a dynamic (100%) setting.
If this is a normal HTML5 project, not a responsive project, then you can just publish with the checkbox on for Scalable HTML Content.
Then the content will attempt to fit into the available browser window space while still maintaining the original aspect ratio.
I also start with a larger project so that most of the time the scaling is occurring down to smaller sizes rather than stretching a smaller project to a larger size. Stetched slides tend to look blurry and of lower quality. Lately I have been building projects that are 1470 x 900 instead of the 1024 x 627.
You don't want to distort a project. When using Scalable HTML either the width or the height will be filled completely. All starts with setting up the correct width/height ratio for the original project. That takes into account which space will be taken horizontally and vertically by the browser or the LMS.
For responsive projects use the Position Properties. Some items cannot be put in a fluid box, like an interactive video. In that case Position Properties will be helpful.