Depends on what you're doing with it. If you're hanging it outside your house I'd go for the bigger one. If you're taking it to pick him up I'd go for the smaller one.
Go for the bigger one if its outside, but only if you know there is space. I work at a copy shop that prints banners and people are sometimes in shock over how large a 3x6 banner is.
Re: What size for a banner?