There are a lot of clothing manufacturers and brands when it comes to active wear. And while sizing measurements are fairly standard, there are differences you might want to pay attention to. Some yoga pant manufacturers have just "one size fits most" because of the extreme stretchiness of the pant. This one size is supposed to be good for sizes 4 through 10. If you're bigger or smaller, you might want to go with a manufacturer that makes standard small, medium and large sizes for a better fit. It's always best to try your pants on in the store, before you buy, or at least save your receipt.
Consider the length of your pants based on the activity and time of year you'll be exercising. If you're heading to a yoga studio, pants that are ankle length or down to your heel are both fine. But if you're going for a run outdoors and it's hot, you'll want a capri-length pant that stops just a few inches below your knee.
Waist and Groin
The two biggest considerations for active wear aesthetically are how it fits your waist and how tight it is in the groin. You want to avoid having a muffin top -- skin that spills over the waist band -- or the dreaded camel toe -- the visible outline of the lady parts between your legs. Pants with a wide waist band do wonders for women with a soft abdomen. Companies have figured this out and make a variety of pants with a wide, fold-over waist band that minimizes the muffin top. If you try on a pair of pants and the camel toe is visible, you should size up -- the pants are too small or not cut in the right proportion for your frame.
Fitted vs. Loose
Not all active wear is tight. A pair of roomy sweats can be considered active wear, too. Base your choice on your comfort level and chosen activity. When it comes to cycling, you don't want anything that's going to get caught up in the gears or pedals, so make sure the bottom is tight and fitted to your leg or ankle. If you're not comfortable wearing skin-tight spandex or Lycra pants while you exercise, choose looser-fitting drawstring pants for a trip to the gym. Be aware that you want to have a full range of movement. So bend over, twist, run in place and do a yoga pose before you purchase a pair of active pants.