This seems more like an XY problem. Why would you want to know the length of a datastore? Do you think there are users in your game losing data because there is simply too much being stored?
There’s no official method to finding the length of a datastore, but you might be able to find out with Crazyman32’s DataStore Editor, I would think.
This isn’t possible since this would require a method to get all of the keys stored in the data store, which is not a feature. The feature on its own probably isn’t feasible because a DataStore doesn’t have a limit, causing a game with a lot of keys, like 1 per player for millions of unique players, to cause Roblox to freeze when creating a Lua version of the multi-million entry list from C++.