It is recommended to use StatementCache in what situation?
What is the value recommended for StatementCacheSize?
StatementCacheSize is the size in bytes or queries?

The StatementCacheSize property determines the maximum number of cached statements when the StatementCache is enabled for the session. The recommended value depends on your application.

For me it is still not clear how "StatementCache" works here and what it is used for.
Is it somehow related to TOracleDataset.cachedUpdates?
We have TOracleDataset.commitOnPost=true and TOracleDataset.CountAllRecords=true.
We see severe network performance degregation and now I would like to understand if "StatementCache" would do the trick that "StatementCacheSize" determines the number of records read in a chunk?
Or do we have to adjust all SQL-statements in the application to limit the amount of records to be read?

It is not related to CachedUpdates. It is an Oracle cache for parsed statements, which can reduce the number of parses for frequently used statements.

One more detail:
is "StatementCache" related to TOracleDataset.ReadBuffer?
(we have TOracleDataset.QueryAllRecords=false and TOracleDataset.ReadBuffer=25)

Which statements are cached with "StatementCache" and does that impact WHEN a post is committed?

Sorry - finally found it explained in the DOA documentation.
"StatementCache" determines if Oracle does cache the statements on the server and thus impacts performance on repeated statements as long as these are in the cache.

Does a change of "StatementCache" require to disconnect and connect again - or can we change this for an active session?

Tried to change this during active session: that is rejected with a message that this cannot be done when connected.