Here we go again!
1) "King" is a Petco marketing term indicating their line of larger-than-standard sized Betta. I have only seen Plakats. Other tail types are extremely rare and not likely to show up at Petco
2) Giants are B. splendens. Most other large wild Betta are mouthbrooders and cannot be crossed with B. splendens.
3) There is a genetic component to giantism (recessive). Absolute size is, therefore not the sole determinant. It is conceivable that you may see a genetic giant smaller than a large regular Betta.
4) The IBC (International Betta Congress) and most breeders measure their fish from nose to caudal peduncle (the start of the tail) not to the end of the tail. To avoid confusion, they sometimes say "BO"---Body Only. I have a 4 inch VT, but his body is two inches long. He's a fair-size fish but no giant.
Here's an article by a master breeder and member of this forum. Its status as a sticky testifies to its acceptance and agreement of other breeders and members.
There's a lot of information about giant Betta online. Need I say more?