Haha I feel you there, especially when working with unity. The way I figure it out is check each screen sizes given by default for unity (Such as 10:4, 5:4, etc.) and set the Canvas to ScreenWithScreenSize(You should see an option like that). And it all comes down to fiddling with the resolution sizes and all. I know that may sound confusing to be honest, but thats how I ended up scaling the size just right for the browser and possibly any screen size XD