Fetch size will vary depending on how you are using SQL App in Incorta.
Incorta over Incorta table: Incorta will send query to Incorta SQL App. Fetch size is controlled by the data source UI for the table. You can specify fetch size in the fetch size field in the data source UI.
Tableau: Tableau will send query to Incorta SQL App. With Incorta connector for Tableau you can specify fetch size in the connector UI in the Advanced tab. Default is 2048. With PostgreSQL connector fetch size may vary depending on Tableau Version.
About size recommendation, I agree with Dawson. 10,000 is the recommended fetch size for better performance.