I try to find the field name for API to get futures contract face value but fail. Can I ask what is the correct way to call ?
The best resource for content search and explanation questions such as this one is Thomson Reuters Helpdesk, which you can reach by either calling the Helpdesk number in your country or by using Contact Us capability in your Eikon application.
To answer your question try a combination of fields LOT_SIZE_A and LOTSZUNITS.
E.g. for Euro BUND future (FGBL) LOT_SIZE_A returns "100,000" and LOTSZUNITS returns "EUR".
For Heng Seng index futures (HSI) these two fields return "50" and "INDEX" signifying 50 Hong Kong dollars per index point.
For Chicago Wheat (W) these fields return "5000" and "BSH" signifying the contract size of 5,000 bushels.