I don't think it's an issue necessarily about size- moreso dogs personality. Jackson loveslovesloves his best little Yorkie girlfriend who is 6lbs and he is 16lbs. They play fine together and can get pretty rough, but she handles it really well and loves every minute of it. However my friends 4lb male is more of a "wimp" and doesn't really get involved in the rough play (though he holds his own!) and doesn't really play that way with Jackson, it's just not his personality. Jackson played with a YT member's foster Chihuahua's who were pretty rough and he loved them! They played great. So I think it's more about finding a personality that fits with your current dog.