first tell me what is step angle of your motor?
is it 1.8 deg, 7.2 deg, 15 deg, 18 deg or any other?
for example if motor is 15 deg then it will rotate 15 deg for each applied pulse. also you have to apply atleast 4 pulse to all four coils. so motor will rotate 60 degree.
as per your requirement you want to rotate motor 15 deg. so you need a motor with 15/4 = 3.75 deg.
another option is choose motor with 7.5 degree. it will rotate minimum 30 degree when 4 pulses are applied.
but if you run it in half step mode then it will rotate for 15 deg.