Answer:
I wear an extra small
Step-by-step explanation:
When using the determiner (an) determine if the following word- in this case extra- starts with a vowel (a,e,i,o,u) either written or spoken. If this is the case use (an). If the following word starts with a consonant then use (a).
I.e., I want to eat an apple. Use (an) because apple start with a vowel.
Has it really only been an hour since we've arrived? You still use (an) because the following word, hour, is pronounced with an (o) rather than an (h).
Hope this helps!