Yes, both the Nominative and Accusative are "mięsa" in plural.
Well, it's a bit more complicated. Firstly, "chcieć" is mostly followed by Accusative, but Genitive is also possible.
Accusative is used for the most basic meanings: "I want X". But Genitive is used in two situations: - for abstract notions (We want freedom = "Chcemy wolności", not "Chcemy wolność") - and as Partitive. Partitive is used mostly with food and drinks, I think. It is used when the intended meaning is "some X". So "Chcesz mięsa?" is like saying "Do you want some meat?" and "Chcesz mięso?" is like saying "Do you want the meat?"