"They have big red tomatoes."
Translation:Mereka punya tomat merah besar.
I would say that in both languages, a string of adjectives is categorized with the closest one to the noun being the most essential discriminant, and the furthest being the least essential discriminant. Usually, but not always, color is considered more essential than size.
Tomatoes are usually categorized by their color-red or green- before anyone starts evaluating their size, so you would talk about a big red tomato rather than a red big tomato. Sometimes though, size is considered more essential than color like if you were talking about a red giant squid, where giant squids is considered a more essential category than red squids.
'Yang' acts like relative pronouns in English. When describing something in English, you would use relative pronouns like 'which', 'that', 'who', etc. In Indonesian, usually you would translate them to 'yang'.
You could also say 'yang mana' to ask 'which...?' or 'which one...?'
- Which one is your house? = Yang mana rumahmu? or Rumahmu yang mana?
- The green one. = Yang hijau.
- Which shirt do you want to wear? = Mau pakai baju yang mana?
- The white one. Yang putih.