That depends if this is a recipe and they need "a cup of milk". http://dictionary.reverso.net/portuguese-english/xicara http://michaelis.uol.com.br/moderno/ingles/definicao/ingles-portugues/cup%20_439544.html
The problem is that in English we might call a glass a cup, but do we really call a cup a glass? I think it might be similar in Portuguese. If a recipe calls for half a cup of milk, you will not be using the Portuguese word "copo" as it is a false friend which means "glass" or container to put a drink in, rather than the measure of 8 ounces in a cup.
I only drink my milk out of a glass. Yes, "copo" would work for that. I use a measuring cup to measure milk for a recipe which is closer in size to a teacup. ("copo" does not work for that? or does it? I thought it was "xicara" for that.) What do you use to measure milk for a recipe?
Ah, here, I found something: http://dictionary.reverso.net/english-portuguese/measuring%20cup Yet the examples all seem to be talking about the kind of small cup that comes to measure medicine or for other medical purposes. I am looking for the kind used for recipes, but then Brazil is on the metric system so they don't measure in ounces or cups.
Here they are using "xícara" for 8 oz cup in a recipe: http://www.ideiasereceitas.com/bolo-de-chocolate-com-cobertura-de-chocolate-2/
I think the US has a standard cup for use with measures. In Brazil using 'copo' or 'xícara' whatever, there is no one with standard measurement. But there's a very common 'copo' all over the country, called 'Copo Americano' http://www.allmix.ind.br/upload/projeto/2010030_gr.jpg then is used as unofficial reference. When you want to spend an accurate measurement using the kilos or liters system, even in the kitchen.
"xícara" or "chávena" is what you use to drink tea or to heat the milk in the microwave.
"copo" is what you normally use to drink water, is made of glass (almost all the times) and does not have an arm (what i mean by the arm is that support for the hand).
P.S.: I hope that i have helped, and i'm sorry for my english... it's still improving!