OK, I have what I believe is a Giant Plakat short-fin male. The one we still have is one of two that we puchased at the same time. What makes me think that he is actually a Giant is that he is 3" long, and over twice as big (thick and wide) as any other Plakat we've had. I do know for sure that he is a Marble, as he has changed colors a few times over the course of the past 5 months or so that we've had him. When we first got him, he was very multicolored, with mostly blue of varying shades, red, orange, and a little black. Now, he's mostly an average blue, with very little other color.

I've read quite varying information on Giants, some saying that a Giant is about 2.75" to 3", while other info' suggests that Giants can get as long as 7 whopping inches! So, what is the real average size of a Giant Betta, and do I have an actual Giant, or is he only half Giant?
Thanks ;)

The whole "Giant betta = 7"" is vague and unrealistic.
IMO, you will never see a 7" PK betta. 7" may be possible in longfin varieties where at least half of the length is fin, but not in PK's.
Even so, 7" is exceptional and most "giant bettas" average 4"-5" for longfin varieties. I would consider 2.75-3" "giant" enough for a PK.
I've personally thrown the term "half-giant" out of my vocabulary. That's just me though. Plenty prefer the gospel of Jim Sonniers.

Now, though I don't have a lot of experience in Guiants, I would say what you have IS a half giant.
I have seen that giants are not only bigger than usual, but they are thicker as well, and that thickness is a sign of giant breeding.,
I cannot say that a 3 inch betta is a giant, though, or even half giant, unless he has the corresponding thickness, because I have had many regular bettas reach 3 inches in length, and the older a betta is, the larger a betta will be.
I agree about the implausibility of the 7 inch betta.
I have heard this number before, but no one has been able to show me proof. I will accept a 4-5 inch rule for giants, with half giants being 3 inches, but thick.
It is not amening Jim's gospel, for me though, it is just convenient to use it to refer to large non giant fish that carry giant genes (those hefty bettas), and to me a 3 inch fish is not a giant, because regulars can get that size.