The size of the eye depends on how you attach your sling to the porty. If you're like Carl and use a shackle/biner to attach it then your eye can be pretty small. However if you're girth hitching the porty directly to the sling I'd say you'll probably want at least an 8 inch eye.
As far as length, it depends on what size trees you have around. If you want to be safe make it longer than you think you'll need.