We have 2 different sets of sizes:

1. For HTML embedded files, we use 20MB for WiFi and 2MB for 3G.
2. When the user clicks to download a module/section we use 100MB for WiFi and 10MB for 3G.

Maybe we want to always use the same sizes in all cases.

Another thing to take into account is that "opening" a file doesn't warn. E.g. in mod_resource clicking "Open" doesn't display any warning, no matter the size of the file, but clicking Download displays the warning.