If you're wanting to retain your current form....
Unless you somehow acquire enough magic items with the drawback of growing six inches to get up to Large, then draw the Mountain Man card from the Harrow Deck of Many Things, then get Enlarge Person cast on yourself and have it made permanent, I don't think so.
I could be wrong, though.
Is there any spell or item that allows you to become bigger then huge?
Mr. Synthesist can. Have the huge evolution and stack on an enlarge person.
In addition to BBTs advice, you could use marionette possession or magic jar as well.
Giant hide armour will allow you to go up to huge, but no, there is no "titan form" spell/item that I know of.
one of the playing in my campaign right now is doing something that allows him to become bigger then huge, but it has more to do with his equipment and skillset combinations then spellcasting. he does it for the purpose of grappling with any size category of creature.
he wears giant fist gauntlets, helm of the mammoth lord and some other stuff. then he uses a SLA that lets him grow 1 size category and has enlarge person cast on him.
after all is said and done, he is gargantuan and his fists make it possible to grapple a colossal creature. this is not permanent however and he can only do it for a few rounds a day.
keep in mind that he also designed the character for the soul purpose of wrestling with an ancient dragon....
Note the following from Animal Growth & Enlarge Person:
"Multiple magical effects that increase size do not stack". (emphasis mine).
Synthesist as noted above is the only one of these options that pass that test, from what I can tell (as evolutions are not magical effects).
EDIT: The possession option is also valid, but I don't think it is what the OP was looking for.
Actually the necromancy spell possess object does it as well. It allows you to get uptown gargantuan. If you can get to 32 caster level you can be colossal.
WISH or MIRACLE
how big do you want to get and for how long? You could have a special mage spell made . . . .
Fail in the use of the Codex of the Infinite Planes and roll between 36-39 on the Codex Catastrophes table and
Colossus: The reader permanently grows to be 65 feet tall, making her Colossal in size. She grows an additional 5 feet every day.
With the possess object idea you can have a gargantuan statue made of adamantium, cast shrink item on it, converting it into a piece of cloth, then pull it out when ever you need some extra muscle.