Most babies born with down syndrome have the same size heads as other babies born without any health issues or defects.
Some babies born with down syndrome may have smaller heads than other babies but most babies that have down syndrome are born with normal size heads.
But the babies head as they grow if they have down syndrome usually grows more slowly than a normal babies head would.
Also babies who are born with down syndrome also often have much less muscle tone so a baby with down syndrome may seem floppy and have more trouble holding their head up but overtime their muscles will get stronger and they'll be able to eventually hold their head up.