Thank you everyone for your kind and helpful words!
ALL the little boys in the litter are precious...they're all so close in size...it could be that they all stay within a few ounces of each other.
The original puppy I picked (the tiniest) was 11 ounces at 6 weeks, and 12 ounces this week.
The one I'm "changing" to was 16 ounces last week, and 18 ounces this week.
They're all eating well and have chubby little bellies...so their weights could of course be not predictable at ALL because of their baby fat. They have the SAME daddy as Sophie...and their mom is smaller than Sophie's mom. Every puppy in Sophie's litter was born 4-5 ounces, and were all very chubby and ate well as babies. Around 6 weeks they stopped growing like weeds...and by 12 weeks they were 16 ounces, 18 ounces, 24 ounces, 30 ounces, and 32 ounces.
Sophie was the 30 ounce one. She hit 2 lbs the next week, then within a month grew to 3 lbs...then by 5 months she was 3 lbs, 5 ounces. And NOW at 6 months she lost her baby fat and went back to 3 lbs. I doubt she'll make it to 4 full grown.
When I picked the smallest one out, I had played with them for about an hour...and I had it narrowed down the biggest and the smallest (so the biggest was actually my 2nd pick). I ended up picking the smallest because the biggest had one ear that was slightly tipped on the end. They were EQUALLY cute and it really took me FOREVER to choose between those two.
I'm just extremely worried about having a "tiny" one. Although, they COULD all be very small, since they're so close. They're still nursing at night, so it could still vary a lot. The breeder and I discussed it today, and she's actually going to do a HUGE favor for me. She's going to hold back BOTH of them and not let anyone else make a deposit on those two until I choose one for sure. She knew it was a tough decision for me...and I have never heard of a breeder being so kind! I've been helping her out the last few weeks with her puppies: washing, feeding, cleaning, etc. because she has a lot on her right now, so we've had a lot of time to talk...and this has also allowed me to spend lots of time with all the boys in the litter...and I'm going back Sunday for the day again!
I just want to make the right choice, and get the little boy who is right for me (and for my girls too). All 4 of my females get along with other dogs fine, BUT if he does end up being very small when I get him, I have 5 crates, an xpen, a playpen, and a baby gate for his safety.
I KNOW that I COULD take care of a really small one, and that's not going to keep me from getting the one who puts pawprints on my heart, but I don't want to make an irresponsible decision because I'm being selfish about getting the one who is the "cutest," you know? I will have a couple weeks off to be with him constantly, but when I go back to work he'll be there 4-6 hours at a time with my others, so if someone comes along who is able to stay home all day or work from home...I would rather they take the smallest, so that I know he's in the BEST possible home for HIM.
Whichever one I end up taking...he will be loved and cared for the same...and he'll have a wonderful home here, and lots of Yorkie friends!!

I want what's best for EVERY SINGLE puppy in that litter...and I want the one that will match with my situation and will fit in the best in my home!!