The entire description of the bladed belt is as follows:
Rather than being made of one continuous line of material, this belt consists of many bits of tanned leather joined together by thick metal studs. On command, the belt’s wearer can transform the belt into a single masterwork slashing and/or piercing melee weapon of her choice. As long as she is holding the weapon, she can also revert it back to belt form with a command. Furthermore, the belt can be enchanted [sic] like a piercing and slashing melee weapon, using the cost of the belt as the cost of the masterwork item.
The typical bladed belt is created with a caster level of 3 using the feat Craft Wondrous Item and the spell magic weapon. It has a price of 2,000 gp and weighs 3 lbs. when it's worn as belt (which makes it even when only a belt a belt not to be trifled with).
So, yeah, you're right in that there is seemingly no limit to the size or variety of masterwork piercing, slashing, or piercing/slashing weapons into which the bladed belt can transform. This GM would be comfortable allowing a bladed belt to transform into, for instance, a Colossal masterwork halberd or a Fine masterwork dagger, but double weapons are a little trickier.
Double weapons and the bladed belt
This GM would be similarly comfortable allowing the weapon to transform into a Medium or bigger or littler two-bladed sword… except that this GM would rule that only one head of the double weapon is masterwork and that, were the bladed belt made more magical, only one head of the bladed-belt-now-double-weapon would benefit from those later magical enhancement bonuses and magic weapon special abilities.
To this GM it seems like the bladed belt shouldn't enable a cut-rate magical double weapon prices just because that magical weapon happened to be first a magical belt! (This GM would rule that the other nonmagical, nonmasterwork head could be made masterwork and magical at the normal prices and would keep these properties upon return to bladed belt form then assuming a different double weapon form.)
The ability transformative and the bladed belt
The magic weapon special ability transformative says, "A transformative weapon alters its shape at its wielder’s command, becoming any other melee weapon of the same general shape and handedness; the weapon’s categorization as simple, martial, or exotic is irrelevant." So for 14,000 gp, a warrior could equip himself with +1 transformative bladed belt, and that bladed belt, upon the warrior's command, could first transform into a +1 transformative longsword then, upon the warrior's second command, that +1 transformative longsword could transform into to a +1 transformative warhammer… if the GM agrees that a warhammer is the same general shape as a longsword.
Limitations of the bladed belt
It should go without saying that the GM will have to rule if a bladed belt can be made from a special material like alchemical silver or adamantine and, if so, then what that means to the weapons into which the bladed belt can transform, but this is incidental to the real issue.
And that real issue with the bladed belt is time.
When the bladed belt says that the belt transforms "[o]n command" and when the magic weapon special ability transformative says that the weapon transforms "at its wielder’s command," it means that each of those command words take a standard action to utter. Magic Items on Command Word, in part, says
If the activation is on command… assume that a command word is needed to activate it. Command word activation means that a character speaks the word and the item activates.… Activating a command word magic item is a standard action and does not provoke attacks of opportunity.
Thus, while that +1 transformative bladed belt is extremely versatile, the wielder pays for that versatility with his most valuable currency: his actions.
For example, the +1 transformative bladed belt's wearer must first be wearing it to take a standard action to utter the command word that transforms it (i.e. "[T]he belt’s wearer can transform the belt…"). (Presumably, this transformation causes the belt-now-weapon to be wielded by the wearer; confirm this with the GM, though!) Then the wielder must take another standard action to utter the command word that transforms the belt-now-weapon a second time. Then, to transform the weapon back into the belt either the wielder takes a standard action to speaks a third command word or—maybe—the wielder takes a free action to drop the weapon whereupon he must take a move action that provokes attacks of opportunity to recover the weapon-now-a-belt. (This latter case is from the ability transformative saying, "When unattended, the [transformative] weapon reverts to its true shape"—a GM may, instead, have the weapon revert to the shape it was when it first became a weapon after having been a belt; get the GM's ruling instead of hoping for belt when you chuck aside your weapon!)
This means that unless the warrior has planned fantastic shenanigans for the +1 transformative bladed belt—and it certainly does lend itself to some!—or the GM allows a bladed belt to made even more magical without the usual necessity of the feat Craft Magic Arms and Armor—which could, in the right campaign, also be a substantial benefit—, most folks will be better off with a regular magic weapon.
Note: You're not the first to notice the many possibilities the bladed belt engenders: here is a 2007 Reddit post, here is an undated Tumblr blog post, and here (2014), here (2012), and here (2014) are some of the many threads discussing the bladed belt on Paizo's own messageboards.