When I cosplayed Aradia(from Homestuck) I had to paint my face, arms, neck and a bit of my legs- and I almost ran out. I got the smallest size and it simply wasn't enough, and it was just for me. I expected it to be bigger, but it wasn't! So I'd maybe think about getting a bigger size, if you want your arms to be covered nicely(mine was not, to conserve the make up).
Next Convention: Anime Detour
Scarlet Witch- Avengers: Age of Ultron
Laurel Lance (Black Canary)- Arrow