My little guy was born with very little body fat, and at 16 months old he's still long and skinny. Most pairs of pants that are long enough to cover his legs just fall off of him! He's destined to be a string bean, by genetics, plus his IUGR diagnosis before birth. He was in size 18 pants for length but size 12 pants for waist and even "slim fit" pants were too baggy! We just tried the Yellow size and it's perfect. I love that there are clearly defined cuffs so I can roll them up if I need to, which he's starting out with. As he grows taller but not much wider, I'll unroll the cuffs and get TWO sizes out of each one before having to size up! They are thick and high quality fabrics that feel soft enough for his skin.