I know spells don't deal with size directly, generally dealing with volume and/or units. However, were you to have a flying and animate longsword, what size would it be? It's easily in the weight range of a cat, which is about -3 to -4; while it's length puts it closer to -2 or -1.
RAW seems to go with dimension more than weight, which is why birds are usually classified as one Size bigger than their weight would warrant, because of their wingspan.
I would agree with Arthur, though birds are actually two categories larger than warranted by their weight, not one. I'm guessing you want the Size for something striking it to damage it. You might look in the Material and Size Tables on page 97 of ArM5. Based on that table I wouldn't make it bigger than Size -2. There are rules in Lords of Men for damaging items, too.
For purposes of damage, the Size of a disembodied sword (or any ordinary weapon) should be 0, as though a mortal was wielding it.
...I have the nagging memory this is addressed somewhere in the rules.