Learn Spanish (Latin America): Ellas tienen una casa grande.

Tienen is the form of the verb tener (to have) used for 'they'. Because the sentence is about ellas (they), we use tienen. If it were about just one person, like she (ella), we would use tiene.

Does ellas mean a specific kind of 'they'?

Yes! Ellas means 'they' when referring to a group of only females. If the group is all males, or a mix of males and females, you would use ellos.

Why is the order casa grande instead of grande casa?

In Spanish, adjectives (describing words) usually come right after the noun they describe. So instead of saying 'large house' like we do in English, you say 'house large' (casa grande).

Since casa is a feminine word, shouldn't grande end in an 'a'?

Not all adjectives change based on gender. Adjectives that end in -e, like grande, are neutral. They stay exactly the same whether they are describing a masculine or a feminine word.