I think size is just personal preference. It may be what we are initially attracted to, but if the pup ends up larger or smaller, doesn't mean we don't love them.
Wid was a rescue, and I don't know what size he'll be. He's about 3 pounds at six months. For me, he's a hearty 'just right'. Now I have a tiny Chihuahua boy who's under 2.5 lbs. and he's full grown, and to be honest, he's a little too delicate for me. I love him dearly, but he requires more diligent attention than my Yorkie.
If I'm buying a toy breed, I chose what I want. If I'm rescuing, it's more who wants me.
Sometimes, too, what we think we want isn't really what ends up working for us sizewise.