Weirdly, cross country spikes normally come with 6mm spike pins in the box. It has to be an in-joke in the running industry because 6mm spikes are the least suitable length for cross country running outside of the USA. So unless you're an NCAA student athlete, you'll almost certainly want something longer.
What length spikes to wear is a question which athletes and coaches debate week in, week out, at cross country races throughout the British winter.
The answer comes down to the course, the recent weather, preference, what length spikes you've got in your bag, whether you arrived early enough to swap them and whether anyone's got a spike key.
Those runners who don't clean their shoes between races may worry that if they unscrew their spikes the holes will be rusted or cross-threaded, or maybe the sole will fall off.
BUT assuming you arrive on time with clean shoes and a spike wrench, this is XCS's thoughts on what's about right for most people and when to use each length:
Nike Waffle
Not spikes, not suitable for cross country at all. Don't be the person who turns up to the first league race with these. It happens.
6mm
Not much in XC terms outside of North America... best saved for the track.
Early season in the UK when it's firm under foot. As a rule, if the ground's too hard to fall on without hurting yourself, it's probably a 9mm spike day.
Younger age groups, who are generally better off going shorter than seniors due to their limb length.
12mm
The standard for most people, if you have one length for the whole season it's 12s. They'll wear down to 10mm soon enough so keep some spare sets to hand to swap in.
September Nationals, if that happens again
Well drained courses throughout the winter.
15mm
Senior age groups running at the end of a full championship programme, especially if it's rained in the last week.
The mud's thick and sticks to your shoes in the car park.
Spectators swap walking boots for wellies and the dogs are wearing coats.
18mm
Extreme filth, the days you'll still talk about in 20 years time.
TLDR - what length spikes do I want
There's a lot of personal preference here but 9mm is right for younger athletes and drier courses. 12mm and 15mm are a good British Standard and 18mm is for when it's "fun".