How many partition(of one datastore) can be created using namespace api?
Namespaces help you increase your app scalability, you have no limit number
Is there any limit on the size of each partition?
App engine Free quota is fixed, it's the only limit. If you need to activate billing, you'll have to fix the limit budget. App engine offers you a very high scalability
how would I know if size of a partition is increased beyond GAE free quota?
As for second question, you have a quota for the whole application, not per namespace. If you consume all the free resources your app will throw an error instead of serving the appropriate handler, until the quota is replenished