Access Order Settings:
On each contract screen, go to more options (+) > Order Settings.
Or in the New Buy/New Sell screen, go to the Settings icon

Edit Order Settings: 
These apply only to the contract you are on, except if you check the "Apply to all" checkbox

Stop Type: Stop/StopLimit. The type of stop you want to use for this contract. Default is Stop.

Duration: DAY / GTC. Default setting is DAY. With DAY, the active order will expire by the end of the session. GTC (good till cancel) will stay active on the following sessions until it is either filled or cancelled. 

Default Size of the order. Useful if you usually trade the same volume. You can still edit it before sending an order.

Max Size: This is a security limit, useful to avoid entering big orders by mistake. If you try to trade more than this limit, it will not let you and will ask you to reduce the volume or edit this setting.