Semiconductor plasmonic Fabry-Perot lasers at submicron and nanometer scales exhibit many characteristics distinct from those of their conventional counterparts at micron scale. The differences originate from their small sizes and the presence of plasma metal in the cavity. To design a laser of this type, these features have to be taken into account properly. In this paper, we provide a comprehensive approach to the design and performance evaluation of the plasmonic Fabry-Perot nanolasers. In particular, we show the proper procedure to obtain the key parameters for lasing action, which are usually neglected in the conventional semiconductor Fabry-Perot lasers but become important for nanolasers.