Standard sizes i.e. S, M, L, XL and One-size only pieces differ for different stores and even different items from the same store. Never assume that just because you usually wear a certain size back home, the same size from the Taobao store will fit you. Size is just a label on Taobao.
The good thing is, the measurements of each size and item will usually be stated in the item's description in cm. Online translations tend to do a decent job with translating them, but if the seller uses images for the size chart, the translator can't pick it up, so knowing what the relevant terms are will be helpful. Here are some common ones:
衣长/衣長： Length of whole outfit / Length of top 裤长/褲長： Length of pants 裙长/裙長： Length of skirt, may sometimes refer to length of a whole dress too 袖长/袖長： Length of sleeve
男： Male size 女： Female size
All sizes are in cm. Measurements stated may not be spot-on due to a variety of reasons. Please expect slight differences of around +/- 2 to 3 cm.
You should thus find out your own measurements in cm first in order to choose the most fitting size. There are lots of guides on taking your own measurements which apply internationally, so I won't go into that. If you really need help, feel free to ask.
Custom sizes for clothing
If you're getting your clothing custom-sized, the basic info required by most sellers is as follows: Height, bust, waist, hips, shoulder width, biological gender. Provide all measurements in cm.
Sellers may specify the measurements they need, so please follow that as much as possible, using the translation I provided above.
Feel free to use your good judgement and common sense to provide additional information. If the outfit has sleeves, then provide your desired sleeve length. If it's a top and a skirt, then provide your preferred length of top and length of skirt. If you often find sleeves tight on your arm, you might wish to provide the circumference of your upper arm. And so forth. There is no guarantee that the tailor will use all the measurements that they did not ask for, but at least we tried.
On the other hand, do not over-complicate measurements. For example, if it's a simple pair of pants, don't split it up into parts and provide thigh length, length of calf and whatnot. No tailor will go to that extent to make it so exact to your body unless it's your personal tailor ^^; Pants length will do. That said, please understand that it is not quite possible to get a 100% exact fit when you have your clothing custom-made online by someone who hasn't seen you in person. If you're particular about the fit, you may have to be prepared to have a local tailor adjust your outfit.
For measurement of body parts, do provide your actual body measurements. The tailor will translate this to garment measurements accordingly so you do not have to incorporate extra space.
Standard sizes for shoes
Shoes on Taobao generally take on Chinese sizes, which are very similar to European size, but usually about 0.5 larger. To get an accurate Chinese size for your feet, you will need to measure the length of your sole.
The upper row indicates the Chinese shoe size while the lower row indicates the corresponding length of sole in mm.
However, if the width of your feet tends to give you issues (e.g. if you have wide feet or bunions), you may wish to provide the width of your feet as well so I can check it out on Taobao's feet size calculator. They have a very good one here but unfortunately you need to be logged in to see it.
Most of us have one foot slightly larger than the other. Make sure you provide the size of your larger foot unless you don't mind it being pinched throughout the day.
Custom sizes for boots
Several cosplay shops that make shoes will accept custom sizing. For boots, you generally need to provide: Biological gender, Chinese shoe size (see above), boot height, circumference of opening in cm. For shoes with no extension beyond the ankle, you'd just need to provide your Chinese shoe size.