Your base size is based on the ready-made Kamakura Shirts.
Take a look at our size chart here.

Measuring your own neck for this is recommended.
Measure your neck tightly, right below your Adam's apple where the collar of your shirt would sit.

Put room for two fingers between your neck and tape, then note the measurement.
The actual collar size will be increased by 0.8 cm in consideration of shrinkage.

Measure on the back side of the shirt, from the center of the back of the collar to the hem of the garment.

The actual sleeve length will be increased by 1.5 cm in consideration of shrinkage.

How to measure:
Measure on the back from the middle of the shirt, at the base of the collar.
Go along the yoke, then pivot from the shoulder down to the outer edge of the cuff.