I don't know what's "best" but if it were me I'd get the 4x6 cards. Their signatures tend to be large with embellishments....like swirls, stars, Chip's teeth, Donald's feet, etc. I think they'd have trouble fitting it onto the smaller cards. JMHO
I would get the bigger ones too. One year, I went to a card store (i.e. like Hallmark) and bought some 4 x 6" cards they had for invitations and used those. I bought several with floral borders, animals, etc. - Micheals & Hobby Lobby used to carry a good supply of these but haven't for a long time. They were inexpensive and looked great in my scrapbook.