和我差不多大的人都是两个字的名字 = Everybody around my age has two characters in their given name. 名字 = given name (name without the surname)

people with the same age .someone's age is the same as your age. here “大”means age. 和我差不多大的人都是两个字的名字 , like western people ,they have name, middle name, family name, we chinese also have years ago, so we have that first word is our family name, we call it 姓, the next word is our own name，名, but we all have , genarally, that is two words, 字，now,we make 名and 字get toghter, it becomes, 名字。 with more and more chinese gowing, people get largely, if 名字just use one word, there will be a lot of people who get the same name, so we use two words as our名字。and , in a large family, the middle name can indicate your generation ,are you clear if I answer this question like this

和我差不多大的人都是两个字的名字 = Everybody around my age has two characters in their name.

means people who are about at the age of me,it emphasizes the people.

和我差不多大的人 means people who are about the same age with me. to further explain a little bit, 差不多 means about, around; 大 means big and sometimes can be used as "old, age". e.g. 我年紀大了. my age is big = I am old. 你多大了? how old are you?/ what's your age?