How do I choose the right layout size?

I've read in the manual about supporting multiple screen sizes. But I can't quite grasp what I'm supposed to do with scaling the layout or screen to make sure multiple browsers/mobile can handle it successfully.

If you're making a mobile game, do you just routinely choose an aspect ratio like 4:3 or 16:9 first? Then, after that, you choose the layout size? And the idea is that whatever layout size you choose doesn't ultimately matter because your aspect ratio is what determines how it displays per browser/mobile platform?

So you could choose 16:9 and then select 1280x720 or 640x480 (or something) and it ultimately doesn't matter, because the scaling takes place due to the aspect ratio?

I'm trying to make a game but keep getting hung up on this part as I'm worried that it may or may not work correctly in a browser vs a mobile screen (it's at 16:9 aspect ratio).

Thanks for any feedback on this.

If you are simply developing for mobile phones, I would say 320x480...basically what all my friends are doing and it seems to scale nicely across a wide range of screens.

What I'm really finding out developing with HTML5 is to keep all of your images very tiny! (I mean in the 1000X1000 pixel size at max!)

I would say 320x480

Thanks, this is very helpful!