The smallest build I achieved was 2.47MB with Unity 2020.3 with builtin render pipeline and brotli compression targeting webgl1. This build includes a simple test scene with some falling blocks and I also didn’t start manually stripping out builtin packages, so I think you can go een a bit lower than that. You can test the build yourself here: WebLoadingTest
And that is the repo to see the exact setup I’m using: GitHub - JohannesDeml/UnityWebGL-LoadingTest: Unity WebGL platform and version comparisons with a lot of nice extras
To get the settings for the min size build, just build the project with minsize, then it will apply all settings that are used for that to the project settings 
