Start with weight, not age. Age labels on baby clothes are rough guides, and babies vary a lot. A 6-month-old can weigh anywhere from around 6 kg to 9 kg depending on the child. If you know the current weight, match it to the size chart. If you are guessing, err on the larger side because a 3-month-old fits into 0-3 months for far less than three months. For more on this, see whether to size up in muslin baby clothes.
Muslin cloth has a gentle give to it because of the open weave, which means a slightly bigger size sits comfortably rather than swimming on a small baby. Epic's clothes run newborn to 4 years. The flat seams mean nothing digs in at the shoulders or underarms, so a little extra room is comfortable, not uncomfortable. If you are shopping for a gift and do not know the baby's weight, the practical answer is to go bigger and include the receipt. For gift-buying specifically, what size to buy as a gift when you do not know the baby's weight covers this step by step.
One more thing worth knowing: muslin softens with every wash and does not shrink significantly when you follow the care instructions (machine wash cold, gentle cycle, tumble dry low or hang, no high heat). So the fit you buy is close to the fit you keep. If you are putting together a first wardrobe and not sure where to start, what baby clothes a first-time dad should actually buy is a practical starting point.