With a wide range of order options, Kraken offers a high level of flexibility for exchanging currency or trading currency pairs. One option we don't (yet) offer is "all or none" (also called "kill or fill"). For now, if your order can only be partially filled, it will be partially filled.
To access the order forms, click "New Order" (under the "Trade" tab on your account screen). Then click "Simple," "Intermediate," or "Advanced" depending on the complexity of the order you wish to create. The "Simple" and "Intermediate" forms are limited to market or limit orders, and the "Advanced" form allows all order types.