Translation:My luggage contains two large suitcases and a small one.
"consists of" rather than "contains" - unless the big suitcase and the small one are inside an even bigger case.
What is the difference between ”un mic” and ”unul mic”? wanted to translate the latter as ”one small one” and it was not accepted.
"Unul mic" is "one/a small one".
"Un mic [...]" is "one/a small [...]" (you have to name the object).
I think in this situation "one small one" would have been a better translation.
Ovidiu is right! Actually the word "mic" is polysemantic... If one just says "un mic", it means "a burger" (in Romania "mic" - pl. mici - are very popular small cilindrical minced meat burgers)
Another distinction to be made here:
unu, unul (f. una) - indefinite pronoun
un, o - indefinite article
unu - numeral