Dang. Such a shame, it would be really cool to have a god sword that's like 20 feet longYou can. It is the Scale field in ItemModelData.xml
One problem with that. Items can only be 16x or 32x. If you scale a sword to 2x, It will have half the detail relative to the size of the item. This means that if you want to make a really long spear, The head is going to be ridiculously oversized.
Ideally there would be a way to set the size of an item texture for SD and HD texture packs. This way if you made a pike that is 2x scale, You could make it be a 32x texture for SD packs and a 64x texture for HD packs. This would have the same detail and pixel size relative to the item size as the normal items.