These leggings have a slight pirate fit: tight around the knee, and slightly looser in the thigh. If you have skinny thighs you are going to get the blouse-out look, if your thighs were built to sustain you while running away from the enemy over mountain ranges then you will just appreciate the extra room.
For best results, take measurements over your underwear.
Bust: Measure under your arms, around the fullest part of your chest.
Waist: Measure around your natural waistline, keeping the tape a bit loose.
Hip: Measure around the fullest part of your body at the top of your legs.
Inseam: Using pants that fit well, measure the crotch seam to the bottom of the leg.
Sleeve: Measure from the center of your back to your wrist with arms held out.