Hey there! I'm happy to help!
A possessive adjective in English is the words like my, mine, your, yours, his, hers, their, theirs, etc.
In Spanish, the word for our is nuestro (or nuestra, nuestros, nuestras depending on the thing you are describing). This word is actually related to nosotros. The first "o" in nosotros kind of into a "ue" because that "o" is accented in the word nuestro.
Since it is an adjective, it has to match the noun. The noun familia is feminine so it uses the article "la": La familia.
So, we will have to use the word nuestra to match this as nuestra is the feminine form of nuestro (switching the o to an a) and it now matches the noun (la familia) that it is describing.
Nuestra familia es grande.
Have a wonderful day and keep on learning! :D