The length of a spike differ by individual plants, but the shortest I've seen was about 8inch. Most are well above 10, most of mine are around 15inch.
Whatever the full length it is supposed to be, only then the bud will begin to grow more and open. So if the bud on your plant is already quite big at 6in tall, then you can safely guess the flower size I think.
Make sure you don't dry out the plant as doing so could cause some issues ranging from malformed flower to bud loss.
Good luck!