This week a customer who had ordered a size 60 suddenly contacted us the next day to change to a size 24. Quite a drastic change. Luckily the tzitzit had not yet been tied on.
Because otherwise it would be a big mess to make changes later, I sent him a detailed explanation of tallit sizing basics:
When discussing tallit dimensions, the terms length and width can be confusing. One man's "length" is another man's "width." (In fact, I once had a customer who was an engineer who tried to prove his definition of tallit length and width is scientifically correct.) In fact, even in halachic works the issue is clarified.
The confusion is compounded by the fact that some people (typically, but not always, Reform and Conservative) imagine a tallit as a fairly narrow shape meant to rest on the shoulders and hang in front; for others, especially Orthodox, a tallit is designed to drape down the back and cover the back, with two corners hanging in front and two in back.
24 and 36 are elongated rectangles. The length is always around 72
inches. It's sort of one-size-fits-all. If you're 5'2" it hangs down
pretty long, probably too long, but your parents want "something you
will grow into." When you're 5'10" it still works. The size refers to
the width. A size 24 is 24 inches wide, a size 36 is 36 inches wide.
That means a size 24 covers your shoulders and a bit of your back, and
wraps around your upper arms; a size 36 will hang halfway down your back
and wrap around your arms at least to the elbows.
Sizes 45, 50, 60 and 70 are worn traditional style, covering the back. Here the key dimension is the height. By height I mean from the edge that rests on the back of your neck, down to the opposite edge, which is parallel to the floor. The width (here "width" is also confusing because the "width" is longer than the "height") is less critical. Because the fringed ends of the tallit are folded up on your shoulders anyway, it doesn't matter as much whether it's 67 inches or 73 inches, etc.
Shown below is a size 60 tallit worn traditional style. The "height" is measured from top to bottom. The "width" is measured from one fringed size to the other.