If you have no idea how big the baby is, size 12 to 18 months is the safest bet. Most newborns sprint through the early sizes faster than parents expect, so a gift that fits at six months or a year gets far more use than one that fits on the day of the shower. Epic ships worldwide, and if the size is genuinely wrong, unworn pieces can be returned within 30 days, so there is a safety net.
Muslin is forgiving fabric. The open weave means a garment that runs slightly large still breathes well and sits comfortably rather than trapping heat against the skin. If you want to understand how muslin baby clothes should fit before you buy, that page walks through what to look for. Epic's clothes arrive pre-washed and with flat seams, so there are no stiff spots that make a loose fit feel sloppy.
One more thing worth knowing: muslin softens with every wash, so a size bought now and worn six months from now will feel even nicer than it does out of the box. If you are putting together a gift bundle and want guidance on what else to include, what baby clothes a first-time dad should buy covers the practical basics well. Epic sizes run from newborn through to 4 years, so there is room to pick something the child will genuinely grow into.