I know it's a tough decision!!
Just go with your heart. I know what you mean about worrying about a little one...I didn't realize how small Emma was until I picked her up, and I was so worried about her size.
But as soon as she opened up, she actually became the bully in my house. LOL
She pushes my 11lb yorkie around. Haha!
We have doggie stairs at the couch and at the bed so that she doesn't jump off of anything. She learned really quickly how to use them, and she will not jump off the couch or the bed...only the stairs
If you want to ensure a certain size though, I would wait until they are a little bit older to actually choose. Will she let you pick when they're closer to six or eight weeks so you can see their personalities and size?