The size in the U/S is equivalent to the size of the official counting with is 2 weeks more than when you conceived. So supposing you did conceived at the correct moment then you should be seeing the size of 9 to 10 weeks. HOWEVER, it could also be that you ovulated late that period or even had double ovulation and you got pregnant two weeks later and everything is fine.
The problem with early U/S is that it's just a snapshot and you cannot see if everything is good until you get the second one and see that baby is growing.
I say all this because I AM EXACTLY IN THE SAME SITUATION :/ and also worried. I was supposed to be 8 weeks, but I had an U/S this Tuesday and the size was only 5 weeks. So either my baby stopped growing 3 weeks ago, or I just got pregnant later.
When did you get a positive test? I got it really really late (2 weeks after missing my period), so it even makes sense to be only 5.5 weeks. However, I will know only next Tuesday when I have another U/S.