Finding output voltage of a MOSFET using its datasheet
The mosfet will have approximately 4-5Ω (we can't be exact because of the current and other factors like manufacturing). Which means the voltage output will be around 2.6V to 2.8V. If an exact voltage is needed, then negative feedback will be needed to control the voltage.
Mosfets in open loop (no feedback) are typically used either in the fully off or fully on state. (also a good idea to prevent mosfets from burning up)
The mosfet will dissipate more than 1W in this condition which will be more than it's rated for and will burn up if operated in the circuit above.
The way to find out what the resistance of the mosfet is through this chart:
The resistance can be determined from the Gate to Source (Vgs) voltage. If you know what the voltage is then you can determine the resistance.