recommended cap 3.3 uF to 10 uF you should be ok, but definitely include the 0.1 uF cap in parallel.
Two things are going in here:
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the 5uF (+/-) cap is the bulk capacitor and is in the circuit to assist in maintaining the voltage when the Arduino is called upon to supply additional current: say for LEDs. Wiring from the Arduino back to the power switch back to the batteries or power supply has a small resistance and therefore there is a voltage drop based upon current drawn by thecArduino.
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the 0,1uF is a bypass capacitor to 'ground out' (short) higher frequencies that may be on the power rail. Such noise can exist because of AC to DC power conversion and regulation, or DC-DC boost/buck converters. If the bulk is a 10uF tantalum, the natural low-Z of that part may allow the designer to omit the 0.1uF without degrading the filtering quality.
Ray