When I am trying to download some of the bigger apps/games, marketplace blocks the download until the phone has WiFi connection available. What is the limit for this? 10, 50, 100 MB?
The limit is 20 MB
When the download size is larger than 20 MB and you are connected on a 3g network you'll receive the "Attention required" message.
If you connect to wifi you can tap the attention required message and press retry.