There is a MT7688AN chip datasheet but, I couldn't find any power consumption documents.
Also it depends on how many peripheral will us used ( Linkit Smart has a USB hos capability ) You can add any usb device ( Bluetooth , Flash memory etc )
Also power comsumption is about CPU load of device. It will not fixed value.
I think , hardware documantation of Linkit duo will help you to decide power input .
You can download Schematic here, page 3 is detailed explanation on power paths. Be careful it wrong connection will demage your board.
They use XC6220B331PR-G tı supply MT7688AN and arduino chip.
IC is 3.3V 1000 mA LDO.
I think , 5V 1000 mA power source is enough for support Linkit Smart Duo.
hope this information will help.