Is there a maximum amount of keys that can be stored in the global state?
Is there a space limitation in bytes in how much can be stored in the global state?
Is there a way to expand a contracts global state after it has been initialized?
You can store up to 64 k/v pairs in global state. The key + the value can be no more than 128 bytes. The key can be no more than 64 bytes but you can use smaller keys to get more space for the values.
@JasonW is there a limit about how many accounts can opt-in to an application?
If there are 1000 accounts, do they each get 16 k/v in local storage?
There is no limit on the number of accounts that can optin to contract. Yes, they each can store up to 16 k/v pairs in local storage. That said currently every account can only optin to no more than 10 contracts. This limit is going to be upped very shortly though 
How much will it increase to?