As a (very) rough guide, the pike should be 16 foot. If you assume your pikeman to be 6 foot tall (they were the biggest and beefiest of the infantry) your pike in 28mm scale should be about 70mm long. It is worth noting that soldiers were prone to cutting of 3 or 4 foot off the end (no not the pointy end) to make it more managable, this practice was frowned on by the officers (who did not have to carry it around the country side). If memory serves their was at least one battle against Irish rebels where they had the advantage as they had not shortened their pikes. If you are looking for a 'campaign' feel you could include a few broken (therefore shortened) pikes. I hope this helps you.