I agree with the first person. While I am not a rat breeder, I have owned pet rats. From PETCO, I purchased two medium-sized female facy rats and I'm assuming they were older, because they were completely unused to human contact. I was bitten on a few occasions--hard enough to draw blood. In any case, I did eventually tame them and they were good little pets. 1 year and 7 months later, the first rat died. It was all pretty difficult, but worth it. I really don't believe size matters, except for the age that size indicates. Hell, I'm 5'9" tall. No matter how "big" the rat, I'm still bigger.
(new person here)I agree with the second person mostly. I have owned both small and large rats. What affects MOST of how your rat, no matter what size, reacts to you is how you treat them. I have had both large and small rats be the sweetest things, but the only one who has been disagreeable no matter what I've done was my small rat, Moonshine. She was awful. She bit me every chance she got and it was almost always hard enough to draw blood despite how tiny she was. The rat I have now, Corrie, is my favorite. She is my first fancy rat and is by far the BEST rat I have ever owned. She is perfectly comfortable with all members of my family after an initial, low-stress introduction, and I have had her a year without ever once being bitten. she is the only rat out of the three that I have owned that I would ever consider a part of my family. Moonshine was hateful and the other one was just sort of dense and not wanting to be sociable. Larger rats have larger brains. Corrie, my large rat, is incredibly intelligent, and unlike the small rats, will almost never go to the bathroom outside of her cage even if left our for three to five hours. She adjusts to all members of the family and is great with children. The smaller rats are best observed and not handled.