Nails come in different shapes and sizes. Solid hardwood flooring of different thickness needs different sizes of nails. The general rule is this,
1-3/4” for 3/4" thick flooring
1-1/2” for 1/2” thick flooring
2” for 1” thick flooring
The diameter is also important. 15, 16, and 18 gauge are the common ones, 15 being the widest and 18 being the narrowest.