Is there an Entity Group Max Size?

Question

I have an Entity that represents a Payment Method. I want to have an entity group for all the payment attempts performed with that payment method.

The 1 write-per-s

Answer 1

No, there isn't a limit for the entity group size, all datastore-related limits are documented at Limits.

But be aware that the entity group size matters when it comes to data contention, see Keep entity groups small. Please note that contention is not only happening when writing entities, but also when reading them inside transaction (see Contention problems in Google App Engine) or, occasionally, maybe even outside transactions (see TransactionFailedError on GAE when no transaction).

IMHO your usage case is not worth the risk of dealing with these issues (fairly difficult to debug and address), I wouldn't use a single entity group in this case.