Microsoft translator Support has sent a response for the above queries as follows
1.At this time the notification for low balances only appear in your Windows Azure Marketplace account. The email notification for low balances is a priority feature that we expect to be included in a future release of Marketplace. If you subscribe to a paid monthly subscription, a new feature for paid Translator API subscribers called ‘Auto-Refill’ was added last week and you may find this to be useful depending on your usage scenario.
If you enable this feature for your paid Translator API subscription, you allow Marketplace to automatically refill your subscription if your remaining balance reaches 10% or less of your monthly subscription limit. More information on Auto-Refill can be found here.
2.You are currently able to translate a maximum of 10000 characters per request, but we recommend keeping each request between 2000 and 5000 characters to optimize response times.