Whether you are taking part in a class indoors or by yourself outdoors, wearing comfortable clothing for practicing Tai Chi is a must. You want clothing that allows you to move without restriction, that is cool in the summer, and keeps you warm in the winter.
I tend to like to wear long-sleeved shirts that fit a little on the loose side. Some of the shirts have Tai Chi designs or logos on them, some are just a plain one coloured shirt. But which ever shirt I wear for practice that day, it has to be comfortable, so the fit is not a distraction to my practice.
Some of my favourite T-shirts are getting worn out! I have had them for years now. They are so comfortable to wear while practicing, that I just do not want to give them up. There are holes in the armpits, and the collar and cuffs are getting stretched and frayed.
Over the years, I have worn many different makes of pants for my Tai Chi practice. I tend to like the fit to be loose in the legs. Preferably cotton, but not always. I used to have a pair of light cotton Kung Fu pants I bought at a local martial arts supply store, pants for hiking/climbing, and lately pants from Lululemon which I received as a present for Christmas!.
Shoes or no shoes. Personally, I like to wear shoes when I practice Tai Chi. I understand that some people like to practice with bare feet, but I tend to find being barefoot can be a distraction for myself (with temperature differences from the rest of my body, to small particles of whatever between my feet and the floor and uneven surfaces of the ground itself).
Indoors I have always liked to wear the cotton soled Tai Chi shoes. They are flat and not too stiff, so the whole foot feels the floor. Outdoors, I have worn a variety of types of shoes over the years. What I try to find, is a flat soled shoe that is fairly light and flexible. One of the best shoes I had was the martial arts shoe that was made by Converse. It had a flat black rubber sole and mesh sides. It was a very comfortable shoe to practice in, and fit my feet which tend to be on the wide side. Unfortunately, Converse stopped making them some years ago.
In summary, whatever one wears when practicing Tai Chi, it will be a personal choice (unless you are required to wear a school uniform or shirts for classes). One must feel comfortable and not distracted by clothing to be able to focus on the task at hand… To practice Tai Chi Chuan.