So I managed to get the right sized cotter pins. Without installing the cranks onto the spindle, I could drop the cotter pin through the hole.
However when I attached the spindle, the pin won't go all the way in. It's only roughly 2/3 of the way in. The nut and washer fits on but The pin head itself isn't flush with the cranks.
Does this matter? I thought it's the taper of cotter pin that prevents the cranks from moving anyway.