I think what you need to wonder, regarding HTML5 games is the ratio quality/size.
A crappy 300 Mo game is a bad idea.
A crappy 1 Mo game is still crap, but downloading 1 Mo is faster than downloading 300 Mo.
You have to consider not everyone can download 100 Mo/sec in the world.
So I'd say keeping the ressources the lower possible, but making them count (nice, meaningful graphics/music. Not just rips off for the sake of it).
As I said, I have a slow connexion (about 60 ko/sec in dld generaly). So if I have to download a big file, it will HAVE to be awesome, or else I'll make it some bad mouth to ear.
I'll be more flexible with a 1 to 5 Mo file.