ICAO Annex 14 aerodromes. Section 3.9.5.
The width of a straight section of a taxiway must not be less than 15m for the A320.
[actually it should be 18m wide for a code letter C which the A320/737 aircraft are, but for a code letter C taxiway that is only intended to serve aircraft with a wheelbase of less than 18 metres, ICAO say the width may be reduced to 15 metres; the A320 wheelbase is 12.6 metres and B738 is 15.6 metres]
So 15m it is, and hope that the airport designer has widened the taxiway a bit at the corners (which they almost always do).