Which size of nail is the largest?

The largest size of nail among the choices given is the 16d nail. The "d" in nail sizing stands for "penny," which is derived from an old method of pricing nails. As the number increases, so does the length of the nail. The 16d nail is longer than both the 10d and 12d nails, making it the largest in this roster.

In general, nails are sized according to their length and weight, and in this context, the 16d nail is commonly used for framing and heavy construction due to its length, which allows it to provide more holding power. On the other hand, while the 10d and 12d nails are used for various applications, they do not match the length of the 16d nail, thus categorizing it as the largest among the provided choices.

It's important to consider that the No. 12 designation represents a different type of sizing approach, typically referring to wire gauge or other specifications rather than length. In this case, it makes the 16d nail the clear largest option in terms of traditional nail sizing.