When pronounced as individual letters, we say "an XL" since the letter X begins with a vowel sound.
We say "an XL". Because the letter X begins with a vowel sound, the abbreviation XL (eXtra Large) takes the article "an".
My brother normally wears an XL.
My brother normally wears a XL.
Remember, the rule for using the article "a" or "an" is based on the initial sound of the following word, not how it's spelled.
An XL is usually more comfortable.
Similarly, we say "an X" and "an extra large".
An extra large coffee mug would be great.