Fashion for Function
The most important consideration when choosing yoga clothes is that you feel comfortable. The next most important issue is that you be able to move freely and not be in a state of constant agitation about whether your clothing is going to shift in ways that make you feel exposed or embarrassed. If, for instance, you are drawn toward looser clothing, like a big T-shirt, because you feel self-conscious in tighter tops, be aware that when you are in an inversion, or even in Adho Mukha Svanasana (Downward-Facing Dog), looser clothing tends to slide and may be more revealing than something a bit more fitted.
My suggestion is that you find a top and leggings that are not completely form-fitting but are not so loose that they disguise or camouflage the shape of your body. The yoga fashion trend of the moment is toward boot-leg or flared leggings, but, speaking as a teacher, it is harder for me to correct someone's alignment if I cannot see the shape of their leg. Ideally, I like to be able to see the ankles (and it is useful for you to be able to look down and clearly see your feet) and to have a sense of the knees, and these are elements that can get lost in looser clothing.
Similarly, it would be optimal if you could find tops that cover your breasts but still give a sense of the muscles and bones in your upper body. Again, I'll refer to the example of the large T-shirt, which can tend to bunch up around the neck in a pose such as Downward-Facing Dog, covering up the shoulders and upper back and interfering with a teacher's ability to address the alignment in this area.
The trick is to find some happy medium, where you don't feel so exposed that you are uncomfortable, but you also get the benefit of instruction that is specific to your body. My final thought in terms of negotiating this slippery slope is that whatever you wear to your first class will work fine, and that over time you will find the items that work best for your specific body and needs. Yoga is a process, and so is getting dressed for it.
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