Mmng23, I will answer this for you the best I can. The clothing items that you usually can buy with out worrying about the size are socks, underwear, shorts, ties, hats. Usually as long as you know your getting them for a large person, just make sure you get it in a large size. Usually clothes items like this will fit because they don't deal with precise measurements, but just measurements to fit an overall size type. Now, as far as guessing someone's clothes size without asking, the best thing to do is to go by a natural scale. Meaning if you know what the status quo is for size of a large person, say for example, a man that's 6ft.1inches and 204lbs., which is usually a large size, and the person that you buying for is a little bigger than this example, then I would go ahead and buy an extra large size. Another tip is to look at the length of thier limbs. If they have long limbs, then you can pretty much assume that everything they were in large or better. If they have short limbs, then medium. The third way is to look at the person and see what type of style of clothes he/she likes to wear. Meaning, if they wear baggy clothes, then the chance is they like clothes a size or two bigger than what they actually wear. If they wear clothes that fits like a glove, then it's probably the same size as the status quo.
Mmng23, this were great questions to answer. Glad to be of assistance.