There are 8 maps, each with 3 sizes (Small, Medium, and Large), and you will need to win on each size on each map. This means 24 victories are needed in total for this achievement to unlock. The easiest approach is to find a partner and take turns winning on each size, and keep good records of which ones you have completed. Skirmish matches against the AI do not count towards this achievement, it must be completed over Live. Your partner can surrender and that will count as a win for you, but if he quits it will not. Ranked or Private matches count toward this achievement.