family is/are

Hi

My family is big/My family are big.

---can I use both?

Sometimes (particularly in British English) we can vary whether we use the singular or plural verb in these cases. It can change the meaning though. Remember that the singular verb is used when we are thinking of the group as a whole single entity. The plural verb can be used when we are thinking of the individual members of that group. I would interpret your sentences:

My family is big. There are a lot of people in my family. The group (familly) is large in size.

My family are big. The people in my family are fat. The individuals(making up the family group) are big...which is a euphamism for fat.

my family is big

I agree with the advice given -- but another possiblity that avoids the is/are decision entirely is "I have a big family." (or, if you want to make it perfectly clear that you are talking about your parents and siblings instead of your spouse and children, "I come from a big family.")