Plural of the surname Matthews

[bectronic]

Do I refer to the four members of the Matthews family as the Matthews or the Matthewses?

[David L.]

The Matthews family.
In casual speech, referring to them, you would say 'the Matthewses' (as pronounced).
To write, the Matthews's (quotes omitted for clarity), you would be writing, "The Matthews's car was stolen." This again would be read/pronounced as 'Matthewses car'