How should a lightweight base layer fit?

A lightweight base layer should fit snugly against the skin like a second skin, with no air gaps or loose areas. This close contact is essential for efficient moisture wicking. If between sizes, size down rather than up. The garment should not restrict breathing or movement, but it should not have any excess fabric that could create air pockets. Look for slim or form-fitting cuts, and ensure the sleeves and legs are long enough to overlap with your mid-layer and socks.