why size of nitrogen decreases and size oxygen increases and finally size of fluorine decreases ?

Nitrogen has half filled configuration  as it's 2p orbital is half filled, which is a stable configuration. So, the size of nitrogen decreases. In the case of Oxygen, this stability is lost and the atomic radius increases due to greater electron-electron repulsion.  Rather its the inter-electron repulsion between the paired electrons in Oxygen that causes the increase in the atomic radii.