We (App Inventor) generally don't put limits on things where we can help it. We may give stern warnings (e.g., if you add more than 10 screens). Most "limits" of App Inventor tend to come down to limits our service provides impose. For example, Google does not allow outgoing connections to send more than 10 MB of data at a time, so that is where the "10 MB project limit" comes from. We do not impose any constraints on the Fusiontables control, but Google might. Please look at the link Taifun posted for details.
Cheers,
Evan