People are always surprised when I tell them that.
「いつもそれを言うと驚かれます。」は
People are always surprised when I tell them that.
のように言えます。
例：
People are always surprised when I tell them I come from a family of seven.
「７人家族だというといつも驚かれます。」
People are always surprised when I tell them ... で「…と言うとみんなに驚かれる」
I come from a family of ... で「私は…人家族です」と言えます。
ご参考まで！
カナダ 
