1) Resistance (For I prevent the passage of current through the base => arduino)
usually 1k-220 ohms is acceptable, But it really should be calculated using the datasheet.
2) The passage of current I2 to the plate.
Depends on the motor windings resistance and the load on the motor (no load<<loaded<<<stall). This information should also be found in the datasheet. You could possibly use a ammeter to measure this and then select a transistor with 2x or 3x the maximum rated current. But the datasheet is usually the safest way to go.
I2, as you have labeled it, is the same current that flows through the current limiting resistor, so you just select the proper value for that resistor to limit the return current through ground. **sorry I read the question wrong**