We've had our 4 lb chihuahua for about a month now, and he's our first small dog. It's been tough getting the sizing right for clothes. Usually even if the measurements sound right, it arrives much bigger than expected.We got the blue #2 in small. Percy is 4 lbs, with an 8" neck girth, 12" chest, and between 9-10" in back length. This has a little room without being too roomy. He's been able to get an arm out while wiggling around during Hugh energy play time, but other than that, it fits perfectly with no issues. He's happy wearing it, and it keeps him cozy in our air conditioned house.It seems to be really well made for the price, and he looks adorable. And it's SO soft!Also, I love that it has some stretch to it - makes dressing and undressing a breeze.