When using the word X-ray, the letter X is capitalized and the word is hyphenated.
The same is true when you are describing a graph’s X-axis.
(And, as a corollary, Y-axis follows the same pattern).
When you are writing about magnification, the letter X is used with no space between the number and the X. Example: “This newfangled camera has a 500X zoom lens.”
However, the letter X is not used in either upper or lower case as a multiplication symbol when describing dimensions. Rather, insert a multiplication symbol [at the top menu, click Insert, then Symbol] and don’t use any spaces on either side of it.
Example: “You just can’t fit a king-sized bed in a 10×10 ft bedroom!”