From what I can tell the candidates for largest character in One Piece are:
1) Laboon, enormous whale of questionable scale at the entrance to the grand line.
2) Surume, the enormous kraken tamed by Luffy was shown to wear the Thousand Sunny on its head like a small hat.
3) Sanjuan Wolf, from pictures posted in the Manga he has been shown to be the size of Navy Headquarters
4) Seakings, some of the seakings shown in the calm belt seamed enormous, and perhaps they aren't even the largest.
5) Island Eater, the island eating goldfish was shown to be several times larger than the giants on Little Garden. Though I'm not sure it's size matches up.
I think Sanjuan Wolf is the largest, but I'm honestly not sure.