Alright, my barefoot is 27.5cm long...but I do have wide feet (10cm) so I always have to size up boots .5-1 full size.
I see. Only foot length is an issue in determining the correct board width. All snowboard boots that have 27.5 cm printed on them will be size 9.5 US. That is not relevant to board sizing however. A 27.5 foot length should never be paired with a wide board. I would suggest that you go with the standard 156 and you will be very stoked!