Maybe Probably Yes
The only limit on wielding a weapon telekintetically is the weight limit of whatever telekinesis effect the wielder is using.
However, see the following line:
...can make a single attack with an unattended weapon or one he is holding, moving the weapon up to 20 feet before the attack. Resolve the attack as normal, except that the weapon's movement doesn't provoke attacks of opportunity....
The bold part implies - but not states - that it should be a weapon normally sized for the wielder.
If that statement is determined to not impede this amusing and fun-sounding idea, I do not believe that there is anything else in the class or rules which would prevent the telekinetic from using as big and bad a chunk of metal as they can find.
Bonus points if you have an array of extra objects for selection choice: giant golf club, a pillar from a holy temple (bonus holy damage, right?), the arm off of a titanic statue, an entire suit of armor, random assortment of fish (long-ago robotech campaign reference), an anvil, an anchor, a bathtub, a small boat, a kitchen sink, and so forth.
Also, as a side comment, the Master of the Unseen Hand is one of the very few "mage" classes that a psionicist can actually and legally qualify for as written, making it one of my personal favorites. I managed to make a D&D version of a Jedi once partly using this class.