I would get a small one tailored to small dogs and cats.  Larger ones may be

too heavy for her neck and may not adjust properly.

MyPembroke is 11 weeks old and weighs 7.5pounds.  We got her 3 weeks ago and she weighed 3.4 pounds.  Her mother is 22 lbs and her father is 24 lbs.   She is happy and active.  I have heard that 8 week old corgis should weigh 8 lbs but I have yet to see one that weighs that much at 8 weeks.