Haven't yet made it to the "twenty club"... According to a pike-growth table I found, a 20-pound pike (9kg) should measure approx. 100 cm (ie 40 inches).
And for a 30 lb (13.6 kg) pike expect a length of 117cm (ie 46 inches). Hats off to those who have caught them... and can proudly wear the appropriate t-shirt.
In may ways I think measuring them (especially pike) would be a better standard - harder to mess up and not dependent on time of year. Really want a 30lb t-shirt!