I did a neck ultrasound and on the report is show: "largest lymph node 1.4 cm", impression: "lymph nodes normal size". how can it be normal size?

Small size: A lymph node under 2 cm is usually not thought to be pathologic on most diagnostic studies. So if the lymph node is under 2 cm the radiologist would usual read that as being within normal limits.

The size: of the lymph node is not the only factor in deciding whether a lymph node is abnormal. The shape of the lymph node, the vascularity, and the cortex thickness are all important in helping to determine whether it is abnormal. That said, a 1.4cm(long axis diameter) neck lymph node would be considered within normal range assuming the other features are also normal.