0-3 months usually covers babies from about 7 to 13 pounds and up to roughly 24 inches, which lines up with the newborn stage. Every brand cuts a little differently, so check the actual height and weight on the label rather than going by age alone, especially if your baby is a big newborn or came early. Epic's sizes run from newborn through 4 years, and if your baby sits between two ranges, our guide on whether to size up in muslin baby clothes walks through when to go up a size.
Muslin is woven loosely, so the fabric has a bit of natural give that a tighter weave doesn't. That open weave is also what makes it breathable and quick to dry, and it gets softer with every wash instead of stiffening up. Because of that give, 0-3 months in muslin can feel roomier than the same label in a heavier cotton, which is worth knowing before you compare sizes across brands; we cover this more in do muslin baby clothes run big or small.
Wash it cold on a gentle cycle and skip the fabric softener, since that coats the fibers and works against the breathability. Tumble dry on low or hang it up, both work fine, and you can check how muslin holds up in a tumble dry if you want the details. If a size ends up wrong, Epic ships worldwide with tracked delivery and takes unworn returns within 30 days, so swapping sizes is straightforward.