See the illustration at the bottom for a guide to where the sling should lay. Remember that this is just a guideline, so go with what feels most comfortable to you.

The overall fit should be snug and baby may look squished but as long as you both are comfortable in the baby sling, then you have the right size. If baby does not seem happy in the baby sling, then get moving as most babies like to be moving around. You can also try putting a rolled receiving blanket under your baby to prop him up until he has better head control. Be sure baby's chin is not pinned against his chest.