I believe and someone please correct me if I'm wrong. But the limit on android used to be 50mb and is now 4gb. That is using expansion files. Something I don't think construct supports yet. So for all intents and purposes, you probably want to stay under 50mb
for desktop it's all about distribution. If you are making a stand alone game. Meant to be downloaded/installed. There isn't really a limit. But for a browser game just consider that on average 1gb takes 10-20 minutes to download. Most people won't be around that long to play a browser game.
12mb is perfectly fine.