If using a typeface with a low x-height at a large point size, wouldn't it have the same result as using a medium or large x-height at a smaller point size? So does that mean x-height is only useful when setting very small (like 6pt) point sizes?
x-height is the height of the lowercase letter x. how many characters fit on a line, and depending on how the type is set, how easily your text can be read. At very small point sizes, a font with a larger x-height is easier to read, everything else being equal.
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Does x-height really matter when choosing fonts in using for readability and legibility?
Yes - fonts with a tall x-height are going to look closer to all-caps, which are not as easy to read, generally speaking. You can get away with a tall x-height in a heading or the like, but readability will start to suffer when using them in large blocks of text.
Small fonts tend to be more difficult to read any way. Combining 6pt type AND tall x-height is kind of a 'double whammy'.
Actually, all three metrics are important to readability and legibility: x-height, type size, and leading. A fourth could also be added: the design of the typeface itself. Some are intentionally hard-to-read, but perfect for specific uses (I'm thinking decorative headlines).
A large x-height font is sometimes harder to read than a smaller x-height font, if set small with the leading (space between lines of type) set 100% (same as type size).
So, add leading to keep large x-height fonts easy to read, and use smaller point-size (or smaller em-size or smaller %).
Small x-height fonts can be set with less leading in larger sizes to make them easier to read.